Intraoperative evaluation is important for successful mitral valve plasty (MVP). We performed a saline injection test and a retrograde cardioprotective beating test (RC-beating test) for intraoperative evaluation. The concept of the RC- beating test is evaluation of residual mitral valve regurgitation( MR) under cardiac beating. A 66-year-old man with severe MR underwent MVP. The P3 chorda was ruptured and we performed quadrangular resection. The saline injection test showed trivial regurgitation. We then performed the RC-beating test and it revealed severe leakage from the posterior commissure(PC). Since the PC had a sclerotic change, another quadrangular resection was performed. Moreover,as the anterior leaflet( A3) was slightly elongated, the region was resected in an obtuse-angled triangle shape and repaired by suturing the edges. The final RC-beating test showed no residual leakage. The RC-beating test is useful for detecting residual mitral valve leakage.