Ochratoxin A (och.A) is a secondary metabolite produced by several species of Aspergillus and Penicillium. This investigation was planned to estimate the quantity of ochratoxin A produced from the isolate (SU) of Aspergillus ochraceus on maize substrate and then to study some histopathological changes induced in rat liver due to ochratoxin A mycotoxicoses. The results are summarized as follows: 1-The amount of ochratoxin A produced by the isolate (SU) of Aspergillus ochraceus was 300 ppb estimated by scanning densitometer. 2-The use of crude ochratoxin A orally in rats or by fed diet contaminated by the spores of this fungus to the rats lead to presence of clinical signs include marked increased in body weight, dyspnea, diarrhea, shivering, increase water and feed consumption, bristling up of body hair and irritation around the neck. 3- The histopadiological study involved the liver by using light microscope after one month of treatment with ochratoxin A and contaminated diet revealed many alterations in liver include lose of hepatic cytoarchitecture, vacuolation in the cytoplasm of hepatic cell, with nuclear swelling (karyomegally), hemorrhage and different degenerative changes in the hepatocytes, abnormal cellular masses in the liver structure with dilated sinusoid were observed, and finally increased number of kupffer cell. 4-Ultrastructural study of the liver by the electron microscope revealed changes in all treated animals, including degeneration in the nucleus and cytoplasm of the cell, and mitochondria, in addition to fragmentation of the rough endoplasmic reticulum (RER) with ribosom depletion were observed.