An evaluation and training system of muscle strength for leg rehabilitation has been developed by using a new conceptual loading machine. This loading machine, which is called MR fluid active loading machine, mainly consists of a newly designed magneto-rheological (MR) fluid clutch and a reversible induction motor. The MR fluid clutch produces passively the magnetic field-dependent transmitting torque almost independent of the rotational speed. Because of this feature, the MR fluid clutch will be suitable for the loading machine of a rehabilitation system from a viewpoint of safety and relief. This system can perform the isometric and isokinetic strength evaluations and the isokinetic strength training. And also, the system has applicability to the Range Of Motion training (ROM training). In this paper, the methods of the muscle strength evaluations and training in this system are described, and the performances of the evaluation and training modes are discussed.