Abstract: This paper proposes a variable structure trajectory tracking control algorithm by using only joint position measurements, which can treat both states and parameter uncertainties. On the basis of investigations on two types of Observers (linear feedback type and sliding mode type), the theorem of this paper gives the sufficient conditions to ensure the asymptotic stability of a closed-loop system composed of robot, observer, and controller. Nonlinear sliding area is substituted for linear sliding surface to make the trade-off between control accuracy and control continuity. The experiment results carried out on a direct driven robot DDR-1, which is developed in our laboratory, verify the validity of this algorithm.