The purpose of this study is to examine the trajectory of early adolescent aggression and to assess the effects of perceived family factors on it. This study investigated the changes in aggression over time and the effects of three family factors, parental attachment, parental monitoring, and family conflict, on the changes using multilevel modeling. The data from Korea Youth Panel Survey(KYPS) of fourth graders who were followed over five years were analyzed. The results indicated that there was a significant average increase followed by a slight decrease in aggression over time; that is, the data followed a quadratic trend. Parental attachment, parental monitoring, and family conflict had significant time-independent effects on aggression, while only family conflict had a significant time-dependent effect on aggression. These results imply that family factors need to be emphasized in planning prevention and intervention programs to reduce aggression during adolescence.