The members of the eighteenth National Assembly of South Korea were elected on 9 April 2008. The Assembly sat from 30 May 2008 until 29 May 2012.[1]
| Date | Constituency | Incumbent | Party | New member | Party | Notes | |||
|---|---|---|---|---|---|---|---|---|---|
| 11 December 2008 | Proportional representation | Lee Han-jung | Creative Korea | Yu Won-il | Creative Korea | Incumbent's election invalidated on 11 December 2008; replaced by the next member in the Creative Korea list. | |||
| 3 March 2009 | Proportional representation | Lee Dal-gon | Grand National | Lee Doo-ah | Grand National | Incumbent resigned on 3 March 2009 upon appointment as Minister of Public Administration and Security; replaced by the next member in the Grand National list. | |||
| 29 April 2009 | Bupyeong B, Incheon | Koo Bon-chul | Grand National | Hong Young-pyo | Democratic | Incumbent's election invalidated on 15 January 2009. New member by-elected. | |||
| Buk, Ulsan | Yoon Doo-hwan | Grand National | Cho Seung-soo | New Progressive | Incumbent lost seat on 12 March 2009. New member by-elected. | ||||
| Wansan A, Jeonju, North Jeolla Province | Lee Moo-young | Independent | Shin Kuhn | Independent | Incumbent's election invalidated on 11 December 2008. New member by-elected. | ||||
| Deokjin, Jeonju, North Jeolla Province | Kim Se-ung | Democratic | Chung Dong-young | Independent | Incumbent's election invalidated on 24 December 2008. New member by-elected. | ||||
| Gyeongju, North Gyeongsang Province | Kim Il-yun | Independent (elected as Pro-Park) | Jung Soo-sung | Independent | Incumbent's election invalidated on 24 December 2008. New member by-elected. | ||||
| 28 October 2009 | Jangan, Suwon, Gyeonggi Province | Park Jong-hee | Grand National | Lee Chan-yeol | Democratic | Incumbent lost seat on 10 September 2009. New member by-elected. | |||
| Sangnok B, Ansan, Gyeonggi Province | Hong Jang-pyo | Grand National (elected as Pro-Park) | Kim Young-hwan | Democratic | Incumbent's election invalidated on 23 July 2009. New member by-elected. | ||||
| Gangneung, Gangwon Province | Choi Wook-cheul | Independent | Kweon Seong-dong | Grand National | Incumbent's election invalidated on 23 July 2009. New member by-elected. | ||||
| Jeungpyeong–Jincheon–Gwisan–Eumseong, North Chungcheong Province | Kim Jong-yull | Democratic | Jong Bum-goo | Democratic | Incumbent lost seat on 24 September 2009. New member by-elected. | ||||
| Yangsan, South Gyeongsang Province | Heo Beom-do | Grand National | Park Hee-tae | Grand National | Incumbent's election invalidated on 23 June 2009. New member by-elected. | ||||
| 3 November 2009 | Proportional representation | Jeong Kuk-kyo | Democratic | Kim Jinai | Democratic | Incumbent's election invalidated on 9 July 2009; replaced by the next member in the Democratic list. | |||
| Proportional representation | Yang Jeong-lyea | Pro-Park | Kim Hye-seong | Pro-Park | Incumbent's election invalidated on 14 May 2009; replaced by the next member in the Pro-Park list. | ||||
| Proportional representation | Suh Chung-won | Pro-Park | Yoon Sang-il | Pro-Park | Incumbent's election invalidated on 14 May 2009; replaced by the next member in the Pro-Park list. | ||||
| Proportional representation | Kim Noh-sik | Pro-Park | Kim Chung | Pro-Park | Incumbent's election invalidated on 14 May 2009; replaced by the next member in the Pro-Park list. | ||||
| 16 July 2010 | Proportional representation | Chung Jin-suk | Grand National | Kim Sung-dong | Grand National | Incumbent resigned on 16 July 2010 upon appointment as Senior Presidential Secretary for Political Affairs; replaced by the next member in the Grand National list. | |||
| 28 July 2010 | Eunpyeong B, Seoul | Moon Kook-hyun | Creative Korea | Lee Jae-oh | Grand National | Incumbent's election invalidated on 22 October 2009. New member by-elected. | |||
| Gyeyang B, Incheon | Song Young-gil | Democratic | Lee Sang-kwon | Grand National | Incumbent resigned on 26 April 2010 to run in the 2010 Incheon mayoral election. New member by-elected. | ||||
| Nam, Gwangju | Kang Woon-tae | Independent | Chang Byoung-wan | Democratic | Incumbent resigned on 7 May 2010 to run in the 2010 Gwangju mayoral election. New member by-elected. | ||||
| Wonju, Gangwon Province | Lee Ke-jin | Grand National | Park Woo-soon | Democratic | Incumbent resigned on 30 April 2010 to run in the 2010 Gangwon Province gubernatorial election. New member by-elected. | ||||
| Taebaek–Yeongwol–Pyeongchang–Jeongseon, Gangwon Province | Lee Kwang-jae | Democratic | Choi Jong-won | Grand National | Incumbent resigned on 28 April 2010 to run in the 2010 Gangwon Province gubernatorial election. New member by-elected. | ||||
| Cheolwon–Hwacheon–Yanggu–Inje, Gangwon Province | Lee Yong-sam | Democratic | Han Ki-ho | Grand National | Incumbent died on 20 January 2010. New member by-elected. | ||||
| Cheonan B, South Chungcheong Province | Park Sang-don | Liberty Forward | Kim Ho-yeon | Grand National | Incumbent resigned on 4 May 2010 to run in the 2010 South Chungcheong Province gubernatorial election. New member by-elected. | ||||
| Chungju, North Chungcheong Province | Lee Si-jong | Democratic | Yoon Jin-sik | Grand National | Incumbent resigned on 12 April 2010 to run in the 2010 North Chungcheong Province gubernatorial election. New member by-elected. | ||||
| 9 September 2010 | Proportional representation | Lim Doo-sung | Grand National | Choi Kyung-hee | Grand National | Incumbent's election invalidated on 9 September 2010; replaced by the next member in the Grand National list. | |||
| 5 April 2011 | Proportional representation | Choi Moon-soon | Democratic | Kim Hak-jae | Democratic | Incumbent resigned on 5 April 2011 to run in the 2011 Gangwon Province gubernatorial by-election; replaced by the next member in the Democratic list. | |||
| 27 April 2011 | Bundang B, Seongnam, Gyeonggi Province | Yim Tae-hee | Grand National | Sohn Hak-kyu | Democratic | Incumbent resigned on 1 October 2010 upon appointment as Chief of Staff to the President. New member by-elected. | |||
| Suncheon, South Jeolla Province | Suh Gab-won | Democratic | Kim Sun-dong | Democratic Labor | Incumbent lost seat on 27 January 2011. No by-election held. | ||||
| Gimhae B, South Gyeongsang Province | Choi Chul-kook | Democratic | Kim Tae-ho | Grand National | Incumbent lost seat on 10 December 2010. No by-election held. | ||||
| 9 June 2011 | Gangnam B, Seoul | Gong Sung-jin | Grand National | Seat remained vacant | Incumbent's election invalidated on 9 June 2011. No by-election held. | ||||
| 11 June 2011 | Nowon A, Seoul | Hyun Gyoung-byoung | Grand National | Seat remained vacant | Incumbent's election invalidated on 11 June 2011. No by-election held. | ||||
| 4 August 2011 | Seongbuk B, Seoul | Kim Hyo-jae | Grand National | Seat remained vacant | Incumbent resigned on 4 August 2011 upon appointment as Senior Presidential Secretary for Political Affairs. No by-election held. | ||||
| 28 September 2011 | Jung, Seoul | Na Kyung-won | Grand National | Seat remained vacant | Incumbent resigned on 28 September 2011 to run in the 2011 Seoul mayoral by-election. No by-election held. | ||||
| 4 October 2011 | Proportional representation | Kim Kum-lae | Grand National | Lee Young-yea | Grand National | Incumbent resigned on 4 October 2011 upon appointment as Minister of Gender Equality and Family; replaced by the next member in the Grand National list. | |||
| 25 January 2012 | Proportional representation | Yu Won-il | Creative Korea | Sun Kyung-sik | Creative Korea | Incumbent resigned on 25 January 2012; replaced by the next member in the Creative Korea list. | |||
| 13 March 2012 | Proportional representation | Kim Choong-joh | Democratic United | Seat remained vacant | Incumbent lost seat on 13 March 2012. | ||||
| 15 March 2012 | Dongdaemun A, Seoul | Jang Kwang-keun | Saenuri | Seat remained vacant | Incumbent's election invalidated on 15 March 2012. No by-election held. | ||||
| 11 May 2012 | Uijeongbu B, Gyeonggi Province | Kang Sung-jong | Democratic United | Seat remained vacant | Incumbent lost seat on 11 May 2012. No by-election held. | ||||