This paper presents a design method of nonlinear controller for semi-active suspension systems with Magnetorheological (MR) dampers having nonlinear properties. A dynamic model of the MR damper, which consists of a nonliear static element followed by a linear dynamic element, is introduced and then a control law based on the integral sliding mode control proposed by V. Utkin and J. Shi is presented. It is shown that the proposed model enables us to reduce so-called chattering, which takes place due to the nonlinear properties of the damper and high frequency switching of the sliding mode control. Although this model includes more modeling error than other rigorous ones in the literature, the use of this model makes it easy to design the sliding mode controller. Numerical Simulations illustrate the effectiveness of the proposed method.