In this study a mesothelioma case observed in a four-year-old female Rottweiler dog was defined. Macroscopically, on the serosal surfaces of the stomach and intestines, numerous masses showing diffuse distribution, greyish yellow in colour and hard consistency were observed. Microscopically, tumour cells were predominantly spindle-shaped with moderate amounts of eosinophilic cytoplasm and ovoid nuclei. These cells was not stained with epithelial membrane antigen (EMA) staining, the epithelial marker, while stained with the epithelial marker pancytokeratin and mesenchymal marker vimentin in the avidin-biotin-peroxidase (ABC-P) method. As a result, tumor case seen in a dog's stomach and intestinal serosal surface was defined as a mesothelioma according to the histopathological and immunohistochemical findings.