Abstract: The lung from a 4 years old sheep with history of respiratory distress due to presence of cauliflower-like masses on its surface was referred to Department of Pathobiology, Faculty of Veterinary Medicine, University of Shahrekord. After gross examination, for histopathological examination of these masses, tissue samples were taken, processed in routine way and stained with hematoxylin and eosin. Microscopically, proliferation of visceral pleural mesothelial cells produced papillary projections on the lung surface. Numerous and different sized clusters of pleomorphic neoplastic cells with hyperchromatic nuclei were seen beneath the papillary projections that had cystic or acinar appearance in some regions. In addition to proliferation of neoplastic mesothelial cells, proliferation of mesenchymal component was observed in different sections. In some regions, the formation of dense collagenous connective tissue was seen too. According to macroscopic and microscopic features, the masses on the lung were diagnosed as mesothelioma.