We report a case of double orifice mitral valve complicating a partial atrioventricular septal defect.The preoperative diagnosis was suggested by a characteristic angiographic and echocardiographic findings but the correct diagnosis of double orifice mitral valve itself was proven by open heart surgery. The smaller accessory mitral orifice was located in the left lateral leaflet and was left untouched. The A-V septal defect was closed with Gore-Tex patch and localized tricuspid annuloplasty was done using Gore-Tex tubular graft for correction of severe tricuspid regurgitation. The anterior mitral cleft left intact had mild mitral regurgitation on postoperative echocardiography.Postoperative course was uneventful and the patient was discharged 2 weeks later.