Background: Nowadays, children and teenagers easily access computer games while parents are unaware of the dangers posed by excessive use of these games. The present study investigated the effect of educational intervention based on the theory of planned behavior (TPB) on primary school boys’ aggression levels. Methods: This research was a randomized controlled trial. The sample size was determined to be 180 considering a confidence interval of 95% and test power of 90% (group 1=90, group 2=90). The sampling environment was a multi-stage cluster with four primary schools from two education districts in Shiraz City, Iran in 2013. The participants were evaluated once before the intervention and then one week and one month after the educational intervention. We used the aggression checklist (12 items) and a researcher-made questionnaire according to TPB theory, which has suitable validity and reliability. The data were analyzed using SPSS software, version 19. Results: There was no difference between the two groups before the intervention. After the education program, the 4 constructs of TPB, including attitude, perceived behavioral control, behavioral intentions, and behavior, increased in the intervention group compared to the control group (P<0.001). Conclusion: According to the findings, it seems that the educational intervention on mothers based on TPB leads to the reduction of aggressive behavior of elementary school students.