Retained placenta is a condition which means all or part of the placenta or membranes are left behind in the uterus during or after the third stage of labor. It is one of major causes of delayed postpartum hemorrhage. Many cases of retained placenta is concerned with placenta accreta, an abnormal adherence of placental villi to myometrium. Recently, the increasing cesarean section rate makes the incidence of placenta accreta increasing. To diagnose the placeta accreta, ultrasonography has been a good traditional method. Today the frequency of MRI for diagnosis becomes more popular for accuracy. Instead of hysterectomy, the incidence of conservative treatment such as uterine artery embolization or methotrexate was gradually increasing, furthermore more patients want to keep their uterus as long as they can. But the disadvantages of conservative teatment are the risk of hemorrhage or infection and then the risk of secondary hystectomy. We present a case of retained placenta accreta. It was diagnosed ultarsound and MRI that was treated by High-intensity focused ultrasound(HIFU) ablation followed by hysteroscopic resection.