Case 1 was a 2-year-old girl who underwent mitral valve replacement with a St. Jude Medical valve for severe mitral regurgitation 14 days following common atrioventricular canal defect correction. The postoperative course was uneventful, but an unilateral thrombosed leaflet of a St. Jude Medical valve was observed 3 times by echocardiography and fluoroscopy. Thrombolytic therapy with urokinase was done each time and the thrombus was successfully dissolved. Case 2 was a 1-year-old girl who underwent closure of ventricular septal defect and mitral valve replacement with a St. Jude Medical valve for ventricular septal defect, severe mitral regurgitation and pulmonary hypertension. Unilateral thrombosed leaflet of the St. Jude Medical valve and poor left ventricular function were found by echocardiography 11 days after the operation. Thrombolytic therapy with urokinase was successfully performed without any complications. Thrombolytic therapy with urokinase was considered to be effective treatment for unilateral thrombosed leaflet of a mechanical bileaflet valve prosthesis in a child. Poor left ventricular function might be one of the causative factors of unilateral thrombosed leaflet of a mechanical bileaflet valve prosthesis.