Aims: Organophosphates such as diazinon (DZN) induce the production of free radicals and oxidative stress. The aim of this study was to evaluate the role of vitamins E and C as antioxidants in reduction of DZN-induced oxidative stress in rat heart.Materials & Methods: In present experimental study, 36 male Wistar rats were randomly divided into six groups; control (corn oil as DZN solvent), DZN (100 mg/kg), vitamin E (150 mg/kg), vitamin C (200 mg/kg), vitamin E+DZN and vitamin C+DZN. 24 hours after intraperitoneal injection, animals were anesthetized by ether, and heart tissue was quickly removed. After tissues hemogenation, superoxide dismutase (SOD), catalase (CAT), glutathione Stransferase (GST) and lactate dehydrogenase (LDH) activities, as well as glutathione (GSH) and malondialdehyde (MDA) levels were determined by biochemical methods. The data were statistically analyzed using Instat 3.3 by analysis of variance (ANOVA) followed by post hoc Tukey test.Findings: DZN increased SOD, CAT and GST activities (p<0.01) and MDA level (p<0.05), while decreased LDH activity (p<0.05) and GSH content (p<0.01) in heart compared with the control group. Administration of vitamins E and C inhibited changing of these parameters.Conclusion: Administration of vitamins E and C as antioxidant decreases DZNinduced oxidative stress by scavenging free radicals.