The protective role of Vit.B12 on neurotoxic effect of diazinon has been studied and conducted on 24 adults male wistar rats were divided equally into 4 groups, the group I was used as normal control; group II was orally given 1/10 LD50 (3.8mg/kg.bw) of diazinon for one month; group III was orally given 1/10 LD50 (3.8mg/kg.bw) of diazinon plus systemic injection (I.M route) of Vit.B12 (4 mg/kg.bw) for one month; group IV was given systemic injection (I.M route) of Vit.B12 (4 mg/kg.bw) only for one month. Biochemically it revealed a significant (P≤0.05) increase in GSH, CAT and SOD values in both Vit.B12 groups (group III and group IV) as well to control group when compared to diazinon group (group II); while the values of MDA and peroxynitrite revealed a significant (P≤0.05) decreases in the both Vit.B12 groups (group III and group IV) as well to control group when compared to diazinon group (group II); moreover, the histopathological results of the diazinon group (group II) revealed a serious neurologic changes included necrotized neurons, diffuse neuronal edema, perivascular edema and hyperplasia of glial cells; while the Vit.B12 groups showed no effective histopathological changes were more or less to control group. There are a little information has been provided concerning the relation between diazinon and Vit.B12,therefore, the current study concluded that the Vit.B12 has a significant protective role against the neurological toxic effects of diazinon on basis of histopathological and biochemical patterns.