OBJECTIVE The study was undertaken to assess levels of aggression, and to determine factors affecting aggression among South Korean middle school students. METHODS A descriptive study was conducted using self-report questionnaires. The participants were 340 girls and boys from two middle schools and 302 questionnaires were used for the final data analysis. Aggression, academic stress, depression, self esteem, decision-making competency, and happiness were measured. Data were analyzed using descriptive statistics including t tests, one-way analysis of variance, Pearson's correlation coefficients and multiple regressions. RESULTS Aggression had significant correlations with academic stress (r = .21, p < .001), depression (r = .43, p < .001), self esteem (r = -.25, p < .001), decision-making competency (r = -.25, p < .001), and happiness (r = -.21, p < .001). Mean score for aggression was 2.49 out of 5. Significant explanatory variables for aggression were grade (t = 4.39, p < .001), academic stress (t = 2.78, p = .006), and depression (t = 5.03, p < .001). The explanatory power of these factors was 26.9%, and this was statistically significant (F = 16.06, p < .001). CONCLUSIONS Findings indicate that depression, academic stress, and grade (second grade) influence aggression. To decrease aggressive behavior, it is necessary to provide systematic and political programs in schools and local communities that can ameliorate negative emotional factors like depression and academic stress. Additionally, development of positive factors such as self esteem, decision-making skills, and happiness in middle school students is important to reduce aggression.