A novel variable optical attenuator (VOA) based on the photonic crystal (PC) waveguide infiltrated with liquid crystals is demonstrated. The PC waveguide decrement is controlled by adjusting rotation angle of the liquid cyrstals orientation vector. And the VOA properties are numerically investigated by using the finite difference time domain (FDTD) method. Numerical simulation shows that the attenuator in the PC waveguide can be changed based on the orientation of liquid crystals by adjusting the applied field. Furthermore, the attenuator lies in 0.5～25.4 dB with rotation angle of the liquid crystals varying from 0° to 90°, the insertion loss is less than 0.2 dB, and the size is in micron dimension.