A global sliding mode controller (GSMC) based on disturbance observer (DOB) is proposed for motor servo systems which are widely used,but whose performance is vulnerable to external disturbances and parameter variations.The initial state of the system is set on the sliding surface by employing an integral switching function,and by applying linear quadratic regulator (LQR) optimal theory,the sliding mode motion is designed to be a optimal one,which minimizes a given performance index and improves the tracking performances.With DOB estimating the equivalent disturbance and using the same amount of compensation in control input,the switching gain only needs to be greater than the bound of the disturbance estimation error,which alleviates chattering greatly.The compound controller consists of an optimal linear state feedback controller (OLSFC),a nonlinear robust controller (NRC) and a DOB.Experimental results show that the proposed compound control scheme possesses better tracking precision,stronger robustness against external disturbances and parameter variations with great chattering alleviation in control input.