The conventional sliding mode control(SMC) technique requires a prior knowledge of the upperbounds of external disturbance to guarantee a robust control performance. This, however, may not be easy to identity in practice. This paper presents a new methodology, sliding mode control with disturbance estimator(SMCDE), which offers a robust control performance without a prior knowledge of the upperbounds. The proposed technique is featured by an integrated average value of the imposed disturbance over a certain period. The proposed technique is applied to the position control of AC servo motor, and experimental results are compared between the conventional and proposed schemes.