In this paper, an intelligent complementary sliding-mode control (ICSMC) system, which is composed of a computation controller and a robust compensator, is proposed for a class of unknown nonlinear systems. In the computation controller design, an on-line constructive fuzzy neural network (OCFNN) with structure and parameter learning ability is used to on-line approximate an unknown nonlinear term of the system dynamics. All the controller parameters of the ICSMC system are on-line tuned in the sense of Lyapunov stability theorem to ensure the system stability. Finally, the proposed ICSMC system is applied to synchronize two chaotic systems with different order. The simulation results show that not only the ICSMC system can achieve favorable tracking performance but also the OCFNN has the admirable property of small fuzzy rules size.