Abstract: Diazinon, a commonly used organophosphorus pesticide, has been widely used throughout the world in agriculture and horticulture to control insects that feed on crops, ornamentals, lawns, fruits, vegetables and other food products. The toxicity of the DZN causes adverse effects on many organs. The purpose of this study was to examine the protective effect of vitamins A, E and C on liver enzymes alanine transaminase (ALT), Aspartate aminotransferase (AST) and Lactate Dehydrogenase (LDH) in rats exposed to diazinon. In this study, male wistar rats were randomly divided into 10 different groups. The groups were administered normal saline, soybean oil (as the solvent for diazinon and fat-soluble vitamins), diazinon, (30 mg kg-1), vitamins E, C and A (100, 500 mg kg-1 and 400 IU kg-1, respectively) and a combination of diazinon with the same dose of each vitamin intraperitoneally i.p.daily for 14 days. Seven days after the final injection, the animals were anesthetized and blood samples were taken. The photometric method was used to measure the activity of the enzymes. The activities of ALT and AST in the diazinon group were significantly higher than that observed in the control group; however, the diazinon/vitamin E, A, C group displayed significant reduction in ALT and AST activities compared to the diazinon group. The lowest level of LDH enzyme activity was observed in the dazinon/vitamin C group and this was statistically lower than the diazinon group. The results of this study revealed that vitamin E, A and C have a potent protective effect against diazinon-induced hepatotoxicity in rats, which may be due to the scavenging of free radicals and increased antioxidant status.