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Photonic crystals (PCs) have great potential in applications for their ability to control lightwave propagation and the possibilities of implementing PC-based optical waveguide into waveguide coupler systems. In this letter, the novel tunable PC waveguide coupler based on nematic liquid crystals is proposed, and its coupling properties are numerically investigated by using the finite-difference time-domain method. The coupling length of PC waveguide couplers with liquid crystals is shorter than the coupling length of coupler without liquid crystals at the same input wavelength. The refractive indexes of liquid crystals can be changed by rotating the directors of liquid crystals. Then, we can control the direction of light propagation in PC waveguide coupler. These results may provide novel application of switching devices in optical circuits.