This is the review article of 21 patients who underwent the rotatinplasty during the period, from Jan., 30th, 1988 to Mar., 30th, 1992. Ten patients among the 21 patients received isokinetic exercise test (with cybex test machine). As the rotated ankle joint of the affected limb acts as the knee joint with the application of the brace, the evaluation of the motion of the affected ankle joint has the importance for the presentaion of the results after the rotationplasty and for the rehabilitation. The results were obtained were as follows: 1. The disease entity of the tested patients was devided as follows; 3 cases of Giant cell tumor, 6 cases of Osteosarcoma and 1 case of Synovial cell sarcoma. Three cases among the 21 patients underwent amputation, afterward. 2. After the rotation plasty, the torque of the affected muscles were weakened about 70% to 80% than the torque of the healthy muscles at the initial tests. 3. Then, the torque of the affected muscles recovered about 15% to 30%, 3 months after the initial tests. 4. The peak torque of the affected ankle as the new knee joint increased from 4.2 ft 1bs to 8.4 ft 1bs at low velocity and increased from 2.8 ft lbs to 6.1 ft 1bs at high velocity. The ratio of peak torque of dorsiflexion to plantar-flexion decreased from 50.1% to 49.2% at low velocity and decreased from 70.8% to 58.1% at high velocity. The decrease of the ratio means that the plantar-flexor is stronger than the dorsiflexor. 5. The ragne of motion increased from 20° to 38° at low velocity and increased from 38° to 42° at high velocity. The physical examination showed that the range of motion of dorsiflexion changed from 7° to -19° and the range of motion of plantar-flexion increased from 32° to 75°, so the axis of the motion of the affected ankle was transposed to the plantar side. This suggested the ankle was adjusted to the new knee joint. These results show that the cybex test is useful in the evaluation of the results after the rotationplasty and the repeated cybex test and exercise promote the result for rehabi-litation after the rotationplsty.