This study was to investigate differences in peer attachment, emotion regulation style and depression perceived by adolescents according to age and gender of adolescents. Also, aim that identifies variables which have influences on adolescents' depression. The subjects were 2279 middle and high school students(male 1150, female 1129) who live in Seoul and Kyonggi. All respondents answered by self-report questionnaire. The scales of emotion regulation style consist of three parts: support-seeking, avoidance/distraction, and active problem-solving. Data were analyzed by two-way ANOVA, t-tset and stepwise multiple regression. The major findings were as follows. First, older male and female adolescents perceived less peer attachment than younger adolescents. Specially female adolescents showed more differences between older adolescents and younger adolescents. Second, male adolescents showed more avoidance/distraction style and active problem-solving style but less support-seeking style than female adolescents. Third, older adolescents perceived more depression than younger adolescents. Fourth, peer attachment, age, avoidance/distraction style, active problem-solving style, support-seeking style and gender had a significant effect on adolescents' depression. In other words, adolescents who had more avoidance/distraction style and support-seeking style, who had less peer attachment and active problem-solving style, who had more avoidance/distraction style and support-seeking style, who had less peer attachment and active problem-solving style, who was older adolescents than younger adolescents, and who was female adolescents than male adolescents perceived more depression. The study results suggest that the degree of peer attachment was the most powerful predictor of adolescents' depression.