The purpose of this study was to examine the relationships among empathy, peer influence, social support and aggression of adolescents. The subjects were 541 students(356 boys, 185 girls) from 3 high schools in Daegu. Data were collected through self-report questionnaires, consisting of Interpersonal Reactive Index(IRI), Peer Influence Scale, Social Support Scale and Aggression Questionnaire. Data were analyzed by Pearson correlational analysis, MANOVA, ANOVA and multiple regression analysis in SPSS/WIN program. The findings are as follows. First, there were significant gender differences in physical aggression and anger. Male students reported higher level of physical aggression and female students reported higher level of anger. Overall, female students reported higher level of empathy, peer influence and perceived social support. Second, the effects of delinquent friends, for male students, but cognitive empathy, for female students, on physical aggression were statistically significant. Cognitive empathy was the most important predictor of verbal aggression and anger for all students, and it has an important implication. Personal distress has a strong influence on hostility for all students, but more researches are needed to explain this finding. Our results showed that there is a strong link between cognitive empathy and overall aggression in male and female adolescents. It suggests that it is very important to improve cognitive perspective-taking ability for preventing and reducing aggression of adolescents.