A 13-year-old, male mixed breed dog was presented to Veterinary Medical Center, Chungbuk National University for diagnosis and treatment of enlarged mass in right inguinal region. On hematological test, monocytes and neutrophils were slight increased, but other hematological values were normal. On radiological finding, opacity and size of mass in right inguinal region were increased. The mass and testis were removed surgically. Removed mass was oval-shpae and bulky with 5.6×7.6×5.2 cm. Histopathological finding of the mass revealed large and round to polyhedral cells and it was diagnosed as malignant seminoma. That was diffuse type seminoma near to malignant tumor without metastasis. During the follow-up for 13 months, patient showed normal contition without recurrence.