This study examined the effects of peer relationships, depression, and aggression on bullying and victimization among boys and girls. The subjects were 364 3rd grade students (boys, 218; girls, 146) and 368 6th grade students (boys, 186; girls, 182), that is, a total of 732 students from three elementary schools. Data were collected on bullying, victimization, peer relationships (mutual friendship, mutual antipathy, and peer popularity), depression, and aggression (overt aggression and relationship aggression) from July 12, 2012 to July 13, 2012. These data were analyzed by means of a chi-squared analysis, t-test, and a logistic regression analysis. The results revealed that there were differences by sex in the case of direct bullying and victimization but no differences in the case of indirect bullying and victimization. Among boys, the factors influencing direct bullying were depression and overt aggression, and the factor influencing direct/indirect victimization was depression. Among girls, the factors influencing direct bullying were mutual antipathy relations and relational aggression, the factors influencing indirect victimization were mutual antipathy relations and peer popularity, the factor influencing indirect bullying was mutual antipathy relations, and the factor influencing indirect victimization was peer popularity. The results of this study showed that the factors influencing bullying and victimization are differences in sex. Finally, the implications and methodology for developing bullying prevention education programs were discussed.