The purpose of these studies was to examine the frequency and stability of relational and physical aggression and their associations with social-psychological adjustment or peer victimization, and how friendships are involved in the relations between forms of aggression and peer victimization in Japanese children. The sample consisted of 452 (Study 1) and 138 (Study 2) children who were in the fourth and fifth grades. Results of Study 1 demonstrated that relational aggression was uniquely and more strongly associated with internalizing adjustment problems than physical aggression. Moreover, Study 2 revealed that relational aggression and physical aggression were stable over a 6-month period and the stability of relational aggression was reinforced by negative friendships (i.e., high levels of exclusivity and friend victimization). Further, the association between relational aggression and relative increases in relational victimization was attenuated by positive friendships (i.e., high levels of intimacy, companionship, and friendship satisfaction). Interestingly, friendships were unrelated to physical aggression and its relation to physical victimization. The age and gender of the children in the two studies were also examined. Cultural and developmental processes involving forms of aggression, friendships, social-psychological adjustment, and peer victimization were discussed.