Background: Aggression is a kind of behavior that causes damage or harm to others. The prevalence of aggression is 8–20% in 3–6 years old children. The present study aimed to assess the effect of training kindergarten teachers regarding reinforcement behavior therapy on preschoolers’ aggression.Methods: In this cluster randomized control trial, 14 out of 35 kindergarten and preschool centers of Mohr city, Iran, were chosen using random cluster sampling and then randomly assigned to an intervention and a control group. All 370 kindergarten and preschool children in 14 kindergarten were assessed by preschoolers’ aggression questionnaire and 60 children who obtained a minimum aggression score of 117.48 for girls and 125.77 for boys were randomly selected. The teachers in the intervention group participated in 4 educational sessions on behavior therapy and then practiced this technique under the supervision of the researcher for two months. Preschoolers’ aggression questionnaire was computed in both intervention and control groups before and after a two-month period.Results: The results demonstrated a significant statistical difference in the total aggression score (P=0.01), verbal (P=0.02) and physical (P=0.01) aggression subscales scores in the intervention group in comparison to the control group after the intervention. But the scores of relational aggression (P=0.09) and impulsive anger (P=0.08) subscales were not statistically different in the intervention group compared to the controls.Conclusion: This study highlighted the importance of teaching reinforcement behavior therapy by kindergarten teachers in decreasing verbal and physical aggression in preschoolers. Trial Registration Number: IRCT2014042617436N1