Successful correction of the partial form of atrioventricular[AV] canal is now readily achievable. The most challenging technical features of this anomaly are the associated mitral valvular deformity and the frequent inability to obtain mitral competence. Residual mitral incompetence after repair of mitral cleft figures importantly in the causes of early and late failure. So, accurate and complete repair of the mitral valve cleft might be important. Determination of residual severe mitral regurgitation after repair is desirable to permit mitral valve replacement at the time. We have experienced one case of severe mitral regurgitation which was occurred 6 months after repair of partial form of A-V canal [ASD primum] in 22 years old virgin. Pre-op. left ventriculogram revealed severe mitral regurgitation in this patient. We preferred to perform mitral valve replacement with St. Jude valve in this patient at March 1984. Post-op. course was uneventful to now for 5 months and we report this case, review and discuss the literatures.