The free functional muscle transfer (FFMT) is the surgical procedure aimed at reconstructing defective or deteriorated muscle function by grafting free muscles including blood vessels and nerves. Since the free gracilis transfer in the facial paralysis was introduced in 1976, there have been many studies and clinical applications of the FFMT in various donor and recipient sites in brachial plexus injury, muscle ischemic contracture, muscle defect after tumor resection, congenital muscular deficit, multiple trauma. When the reconstruction of the nerve is delayed for 9 months to 1 year after the brachial plexus injury, voluntary muscle contracture is impossible even after the nerve regeneration by the irreversible degeneration of the muscles. And it is difficult to obtain adequate function even if nerve transfer or nerve transplantation is performed because the distance to be regenerated is too long. In these cases, the FFMT has been improved the functions of the upper limb in flexion or extension of the elbow, fingers. Many good clinical results of the FFMT have been reported, so the FFMT gets much interests and attentions. The essential things for the successful outcome of the surgery are the anatomical knowledge, the skilled surgical technique and the choice of patients who can meet the indications and receive long-term rehabilitation. Recent advances in surgical techniques will result in improved results of the FFMT.