Nowadays, the using of robots in the field of rehabilitation has been increased, vehemently. Rehabilitation robots are designed to assist people who have physical disabilities. Patients often have the substantial limitation in movement. The knee joint as the largest blockage in the human body is always vulnerable to injury, some knee joint rehabilitation treatments are provided by physiotherapists by practicing the patient's leg around the knee joint to strengthen the knee-strengthened muscles to the patient gets his health. Most exercises that physiotherapists do is either manual or traditional way. Therefore, researchers are interested in designing a device to help these exercises. In this regard, devices were designed and built, such as Continuous Passive Motion (CPM). The main problems of CPM devices are lack of knowledge (feedback) from the position of the patient's leg and its resistance against the robot motion. Therefore, interactive forces between patient leg and robot will be increased if the patient's leg is unable to track the predetermined trajectory. This phenomenon can hurt the patient. In this thesis, in order to realize the flexible behavior of the robot against the potential force of the individual foot, the concept of admittance as well as impedance are used. At first, the dynamic equations of the robot and the patient leg were extracted and verified using Adams dynamic analytic software. Then, the design of nonlinear controllers is done. In this research, a variety of control methods including a combination of Sliding-Backstepping and Admittance Control, in order to control the knee rehabilitation robot and at the same time creating a soft interactive patient with the patient leg are developed.