This paper proposes a new H_∞ approach to chaos synchronization for Duffing oscillator in the presence of external disturbance. Based on Lyapunov stability theory and linear matrix inequality (LMI) formaulation, the proposed synchronization method is presented to not only guarantee asymptotical synchronization but also reduce the effect of external disturbance to an H_∞ norm constraint. A simulation study is given to demonstrate the effectiveness of the proposed method.