Objectives: This study aimed at investigating the effectiveness of the Olweus Bullying Prevention Program (OBPP) on the rate of bullying behaviors among primary school boys. Method: This research used a pretest posttesting design and 6-month follow up on subject and control groups. Using randomized multi-stage sampling, one district was chosen out of six districts of education in the city of Isfahan. Four boys primary schools were randomly selected from that district. Sociometric questionnaire was applied to all boys in grades 2-4 and 40 boys who scored highest as bullies were randomly allocated to experiment and control groups. The experiment group received 10 sessions (60 minutes, twice a week) of anti-bullying training based on the OBPP. Rea search data were collected using sociometry and Peer Relationship Questionnaire. Findings were analysed using multivariate analysis of covariance. Results: The experiment group had significantly lower bullying and victim behaviors at posttest and 6-month follow up (p<0.0001). Also the results showed significant improvement in prosocial behaviours at posttest and 6-month follow up (p<0.0001). Conclusion: Anti-bullying training programs based on the OBPP can reduce the bullying problem at schools.