A female, 14-year-old mixed dog weighing 4 kg and a female,8-year-old schnauzer dog weighing 6 kg showing vaginal discharges and vomiting for a few weeks were examined for the preoperative check of anterior cruciate ligament rupture and the recheck after the surgery of nephrectomy. There were no distinct abnormal signs except for the increased CBC at schnauzer dog on blood chemistry analysis. Radiographic and ultrasonographic findings were a large abdominal mass and a cyst-like hypoechogenic or anechogenic cavitary lesion on the ovary region. These findings were consistent with an ovarian cyst. Thereafter highly suspected ovarian cysts by diagnostic imaging were supported by high estrogen concentration on hormonal assay and the results of Pathologic examination as the unilateral ovarian cysts with secondary pyometra.