The purpose of this study is to identify the mediating effect of pro-social behavior and defending behavior in the relationship between sense of peer community and overt aggression in a sample of students of Grade 4, 5, and 6. The fourth wave of Class-Net data was used to target a total of 1,374(girl 50.0%) elementary school students. Sense of peer community and pro-social behavior were measured by self-reports whereas overt aggression and defending behavior were measured by peer nomination. The major findings through using a SEM were as follows. First, sense of peer community had a positive effect on pro-social behavior and pro-social behavior had a positive effect on defending behavior. Second, sense of peer community had a positive effect on defending behavior and defending behavior had a negative direct effect on overt aggression. In addition, sense of peer community had a negative effect on overt aggression. Third, defending behavior mediated partially in the relationship sense of peer community and overt aggression, defending behavior mediated partially in the relationship pro-social behavior and overt aggression too, Also, pro-social behavior mediated partially in the relationship between sense of peer community and defending behavior. Finally, sense of peer community had a partial mediation effect on overt aggression through pro-social behavior and defending behavior. Based on these results, it is indicated that acquiring defending capacity was necessary to reduce overt aggression, and defending behavior is an important variable to prevent school bullying, which appears in the form of overt aggression in the school violence context.