Abstract: In order to enhance the locomotion adaptability of legged robots, a novel compliant rotational joint with continuously adjustable stiffness is designed. To achieve this, the relation between transmission ratio and output stiffness of lever mechanism is analyzed, and the lever mechanism with variable transmission ratio is utilized as a core functional module in the design of the compliant joint with adjustable stiffness. The joint is designed to be light weight and compact in size, and the joint's mechanisms as well as driving module are specified for practical applications to legged robots. The relationship between joint structure parameters and output stiffness is analyzed, and the range of the relevant parameter is selected so that the output stiffness can be sensitive to the parameter change. The natural stiffness characteristics of the joint are further determined based on kinematic study. Tests on the prototype joint show that the output stiffness can be adjusted and controlled effectively by the adjusting-mechanism, and the joint can be applied to robotic legs with respect to its structure design and functions.