Introduction: Attention-deficit/hyperactivity disorders (ADHD) affect ~ 5% of school –aged children. This condition is associated commonly with difficulties in learning, behaviore, and psychosocial adjustment that persist into adulthood. Mounting evidence suggests that a lack of certain polyunsaturated fatty and stress oxidatie may contribute to neurodevelopmental and psychiatric disorders such as ADHD.Methods: In this clinical trial study, 100, 6-12 years old, children with ADHD under effective maitenance therapy were assigned randomly, in a double-blind-fashion, to receive Omega-3 fatty acids (635 mg Eicosapentaenoic acid (EPA), 195mg Docosahexaenoic acid (DHA)) or placebo for 8 weeks. Outcome variables included activity of enzyme gelotatiun redoctase (GR), catalase (CAT) and superoxid dismutase (SOD) and abbreviated conners 10-item parent rating scale (CPRS), before and after supplementation.Results: After 8 weeks supplementation a significant difference was observed in activity of SOD, GR enzymes (P<0.05) but there was no significant difference between groups at activity of CAT at the end of intervention. Furthermore, a significant improvement was seen in CPRS score in omega-3 group.Conclusion: This study suggested that fatty acids supplementation may offer a safe efficacious treatment option for hyperactivity problems in ADHD and reduction of oxidative stress.