11 year old male Yorkshire terrier was referred to Haemaru Referral Animal Hospital with signs of hematuria, petechia, and gynecomastia. Blood works revealed severe leukopenia, moderate anemia and severe thrombocytopenia. On ultrasonography and radiography, mixed echo texture mass was found in abdomen. The abdominal mass was surgically removed, and submitted for histopathology. Histopathologic features of the tissues were consistent with malignant Sertoli cell tumor. Bone marrow aspirates were hypocellular. Serum estrogen concentration was 72.80 pg/ml (normal range for females <15 pg/ml) after surgery. Clinical signs of feminization and hemorrhagic diathesis were attributed to hyperestrinism caused by the tumor. The dog was put on fluid therapy, antibiotics and palliative drugs and survived 2 more weeks after surgery without clinical improvement.