In this paper, we discuss the accurate and robust sliding mode tracking control for highly nonlinear robot manipulators using a disturbance observer. Due to the modeling error or environmental uncertainties, sliding mode control may present a significant chattering problem by using a conventional Sliding Mode Control (SMC) which can obtain the desired tracking performance, because the controller design is carried in the uncertainty space of the system parameters. To solve this chattering problem, the efficient compensation of the disturbance observer has been introduced. Thus the design of the proposed SMC is not influenced by the modeling error or parameter uncertainties. To ensure bounded stability, the proposed sliding mode controls have been analysed in a preliminary lemma and two main theorems. The stability of the proposed control method is proved in this paper, and the efficiency of the control algorithms has been demonstrated by simulations for a position tracking control of a two-link robot subject to parameter and payload uncertainties.