Early postnatal experiences have been shown to have a tremendous influence on behavior development. In this study, we focused on weaning time and investigated the effects of early weaning on adulthood aggressive behavior in mice with the resident-intruder test. Early weaning resulted in decreased sideways threat and tail rattle. In addition, the frequency of attack bites was more variable in the early weaned group, although the mean frequency did not differ from the normally weaned group. The results of the present study suggest that aggressive behavior is also affected by early weaning manipulation.