Purpose: The purpose of this study was to explore suicidal ideation among adolescents and to identify the factors that may increase the likelihood of suicidal ideation among adolescents. The factors included depression, individuality, relatedness, cognitive deconstruction, social desirability, and alexithymia. Methods: The data were collected from March 4 to May 29, 2009 from 1,519 adolescents who were conveniently selected from five different middle and high schools in Seoul. The data were analyzed and interpreted using descriptive statistics, correlation, and stepwise multiple linear regression with the SPSS/WIN program. Results: The major findings of this study were: 1) More than 30% of the participants had a history of having suicidal ideation while 6.3% had attempted suicide. 2) Suicidal ideation was positively related to depression, alexithymia, and cognitive deconstruction and negatively related to relatedness and social desirability. 3) Regression analysis showed 29% of variance in suicidal ideation can be explained by depression, individuality, cognitive deconstruction, living with father, living alone, and living with relatives. Conclusion: Adolescents with depression, higher individuality, cognitive deconstruction, and not living with their parents require more attention to prevent suicide. It is necessary to develop programs to prevent adolescents` suicide.