Polioencephalomalacia caused by thiamine deficiency in sheep of Barash province have been diagnosed. This current research was conducted on 44 local sheep breeds 1-3 years old and of both sexes reared in Basrah province. Ten clinically normal local sheep breeds were selected as controls. Diseased animals show various clinical manifestations, Such as Anorexia and head pressing, Muscular tremors. Moreover, diseased sheep show signs of opisthotonus and nystagmus, incoordination and ataxia, Furthermore, 41% of the diseased sheep show circling with lateral deviation of the head, recumbency and inability to stand. However, 18.1% of the diseased sheep exhibit blindness. Moreover, significant increase was detected in respiratory and heart rate of diseased animals in comparison with controls, but ruminal motility was decreased significantly in affected animals compared to controls. Additionally, considerable changes were detected in biochemical parameters of affected sheep in comparison with controls. The results of cross pathological features and histopathological examination of dying or slaughtered sheep show pale discoloration of the dorsal cerebral cortex, especially the occipital lobes, which might indicate a necrosis. However, gross lesions might also represent flattening of cerebral gyri, hemorrhages, necrosis to cavitations in the gray matter of the occipital and temporal regions. Further, histopathological examinations revealed gliosis as well to vacuolative changes of some neurons, Moreover, sections of the white matter of brain showed also gliosis with wallerian degeneration of some neuronal axons, Furthermore, sections were also indicated a severe vaculation of neuronal cell body and central chromatolysis of some neurons.