A 10-year-old mixed male dog, weighed 4.2 kg, was referred to an animal hospital in Pusan. Clinical signs were generalized alopecia, cough, cardiac murmur, dehydration, and right side cryptorchidism. Testis was surgically removed, fixed in formalin and submitted to Diagnostic Laboratory, Chonbuk National University. Grossly, right testis was enlarged. A bulging tumor mass of right testis was homogeneous and grayish white in cut surface, but left testis was normal. Microscopically, no border lines of tubules were formed and the cells grew diffusely, forming sheets with scant supporting stroma. Tumor cells were flirty uniform in size and round or polyhedral, and had discrete cellular lines. The nuclei were large and of variable size, and ovoid, round, vesicular but the cell cytoplasm was scanty. Mitotic figures were common. And giant cells and vacuolated histiocytes were scattered, which is called starry sky appearance. This is the case of seminoma with diffuse type in a mixed dog.