In this dissertation, a tendon-driven robotic joint with a variable torque mechanism is proposed to solve the problems of hardware of existing robot manipulator. The mechanism concept and design method of a variable-radius pulley that passively changes its transmission ratio as a function of the external load is presented. The key mechanisms of the proposed variable-radius pulley are the face cam mechanism and the sliding mechanism of the follower, through which the follower from the pulley increases the pulley radius. Because the internal stiffness of the rotating spring acts on the rotation of the face cam, the rotation angle of the face cam changes according to the output load, and the sliding length of the follower changes according to the rotation angle of the cam body. The relationship between the output load and the radius of the pulley, which is equivalent to the transmission ratio, can be tuned to optimize the torque capacity region of the actuator, which is used for a tendon-driven robot manipulator. After describing the working principle of the new variable-radius pulley, static analysis of the proposed mechanism is presented. Simulations and experiments are conducted to demonstrate the proposed mechanism, and the results show that the proposed variable-radius pulley can be used to vary the transmission ratio according to the torque applied to the robot joint.