This paper aims to solve the sliding mode control (SMC) problem for systems with mismatched uncertainties via a disturbance observer. By designing a novel sliding surface based on the disturbance estimation, a disturbance observer based sliding mode control method is proposed to counteract the mismatched disturbance which is possibly nonvanishing. There are two distinct features for the proposed method. Firstly, the switching gain is only required to be designed greater than the magnitude of the disturbance estimation error rather than that of the disturbance, thus the chattering problem is substantially alleviated. Secondly, the proposed method retains its nominal performance, which means the proposed method acts the same as the baseline controller in the absence of disturbance. Application to a MAGLEV suspension system shows that the proposed method exhibits much better control performance than the baseline SMC and the integral SMC (I-SMC) methods, such as reduced chattering and nominal performance recovery.