Sertoli cell tumor (SCT) of the testicle arises from the supporting cells within the seminiferous tubules. SCT is common in dogs, especially in cryptorchid testicles, but also has been reported in the stallion, ram, cat, and bull. Sertoli cell tumor sample was collected from 7-years male german shepherd. In this study, SCT arose from one testicle. Sample size is approximately 1.7 cm in diameter and it has a round form. In the microscopic, cells within the tumor variably resemble Sertoli cells (SCs) that normally populate the seminiferous tubules and interstitial area. There is abundant stroma of dense, mature fibrous connective tissue in SCT. In the immunohistochemical staining, cytokeratin AE1/AE3 was not expressed in the control and SCT. S-100 protein was expressed by SCs, germ cells and fibrous connective tissue of SCT. Melan A was expressed by leydig cells (LCs) of SCT. A study by using S-100 and melan A in canine SCT was almost never carried out. S-100 and melans A is considered to suggest for diagnosis and pathogenesis of canine SCTs. Inhibin-alpha and Vimentin were well known as the marekers of SCTs. Also, they were expressed by Sertoli cells and LSs of SCT in this study.