Objective: This study aims to integrate the results of different psychological treatments studies for Attention-Deficit/ Hyperactivity­ disorder (ADHD), and to determine the effect size. Method: Seventeen studies with appropriate methodology were selected and meta-analysis was done on it. The research tool was meta-analysis checklist. Results: Data analysis showed that rate of effect size of psychological intervention on reducing ADHD symptoms were 0/54.(P≤0/001). Conclusion: This rate of effective size according to Cohen table was higher than average.