This study aims to investigate peer socialization processes on early adolescents’ prosocial behaviors in friendship networks. A longitudinal social network model (SIENA) was used to disentangle socialization process in prosocial behavior. Participants were fourth and fifth graders in South Korea from 39 elementary school classrooms (N=1,040, 48% girls). According to results, students select friends based on similarity in prosocial behavior (peer selection) was found in both prosocial behaviors. Peer influence effects were also observed in prosocial behaviors. Also, moderating effect of social relatedness was significant. In classes with high social relatedness, influence effect of prosocial behavior was significant. As a contrast, prosocial behavior was not socialized in classes with the low social relatedness. Results suggest that peernetwork play a critical role in the co-evolution of friendships, and prosocial behaviors. Findings yielded the importance of the socialization process and promoting healthy peer socialization environment.