This paper focuses on the problem of disturbance compensation in motion control. Disturbance observer (DOB) and sliding mode control (SMC) are both famous approaches to solve the problem. However, the DOB does not deal with nonlinear disturbance sufficiently. The SMC has chattering phenomenon which limits its application. In this paper, the idea of sliding mode assist disturbance observer (SMADO) is proposed, which avoids the shortness and associates the advantages of both SMC and DOB. In the proposal, the DOB helps the SMC to decrease the switching gain, which weakens the chattering. On the other hand, the SMC assists the DOB in compensating nonlinear disturbance. Based on the basic design, some improvements are also given. To weaken the chattering further, the smooth design is implemented. Moreover, to further systematize the idea of SMADO, the systematical adjustment method of the switching gain is investigated for the basic and smooth SMADO, respectively. The designs are proved by Lyapunov-based method. The validity of the proposals is confirmed by experiments.