In order to estimate the hepato-protective activity of Vit.B₁₂ in diazinon hepatotoxic effect, the current study was performed on twenty-four male adult Wistar rats. They’d divided into four equal groups for 30 days: group A (control group) served as control; group B (Dzn group) administered an oral daily dose of 1/10 LD₅₀ (3.8mg/ kg.bw) from diazinon; group C (Dzn + Vit.B₁₂ group) administered an oral daily dose of 1/10 LD₅₀ (3.8 mg / kg bw) from diazinon, in addition to the systemic intramuscular doses of (4 mg/kg) from Vit.B₁₂ in daily basis; group D (Vit.B₁₂ group) administered daily systemic intramuscular doses of (4 mg/kg) Vit.B₁₂ only. The results showed that group C had a significant improvement in the antioxidant indicators compared with group B in which the CAT, GSH, and SOD serum activities showed significant increasing (P≤0.05) in group C, in addition, it revealed a significant decreasing (P≤0.05) in the values of MDA and peroxynitrite comparing to the same levels of the group B. The histological results of the liver of group C showed a great improvement compared with group B which showed normal hepatic architecture consisting of normal hepatocytes, and normal sinusoids with normal other hepatic components except mild central vein dilation. The current study concludes that vitamin B₁₂ helps to ameliorate the diazinon hepatotoxic effects as a model for a wide spread of organophosphorus compounds in agriculture and veterinary sectors by ameliorating its harmful oxidation on the liver since vitamin B₁₂ is cheap and available as a commercial supplement therefore we recommend its uses