A cleft in the anterior mitral valve leaflet is commonly associated with an atrioventricular septal defect, but an isolated cleft mitral valve is a rare condition. We report a case of a 27-year-old woman with severe mitral regurgitation due to an isolated cleft mitral valve. The echocardiography showed an anterior cleft on the mitral valve, but a normal sized and positioned left ventricular papillary muscle without septal defect. Under the diagnosis of mitral regurgitation due to the isolated cleft mitral valve the patient underwent mitral cleft repair. After surgery, the further echocardiography showed no mitral regurgitation.