This paper proposed an active sliding mode method, which combines the active control theory and the sliding mode theory to synchronize fractional-order chaotic systems. In the active sliding mode control strategy, first the fractional-order integrator was introduced to obtain a novel sliding surface containing all the state components in the system, and the fractional order chaotic system is stable in the sliding surface. Then the gain matrix of the active sliding mode controller was obtained using the pole placement method. The existence and the stability of the proposed controller were analyzed based on Lyapunov stability theory and fractional stability theory. The simulation results of the fractional order Lorenz system verify the effectiveness of the proposed method.