A modified Euler angles method, dual Euler angles approach, has been proposed to describe exact joint motion. In the dual Euler angles method, joint motion is considered as three successive screw motions with respect to the axes of the moving segment coordinate system and the screw motion displacements are represented by dual angles accordingly. The algorithm for calculating dual Euler angles from coordinates of markers on joint segments has also been provided. In this paper, the dual Euler angles method is applied to describe the motion of ankle joint during dorsiflexion-plantarflexion. Due to the difficulty in tracking the motion of the talus segment in vivo, only the overall motion of the ankle-subtalar complex, that is, the relative motion of the foot with respect to the shank, was measured in the present study.