Cleft of the post mitral valve leaflet is extremely rare congenital cause of mitral regurgitation. Only six cases have being reported in literature. We report the case of a 34-year-old man with isolated cleft in the posterior mitral valve leaflet causing severe mitral regurgitation. The patient had been experiencing limitation of his exercise capacity becoming more easily tried. Therefore, he was referred for mitral valve surgery. The posterior mitral valve leaflet was successfully repaired using quadrangular resection technique. Intra-operative transesophageal echocardiography showed trivial mitral regurgitation after repair. Satisfactory clinical results were achieved at 2 years post-operatively.