Abstract: A novel method to control the photonic band gap of 2-D photonic crystal is proposed. The two-dimensional photonic crystal, which composes of triangular dielectric cylinders between which liquid crystal (phenylacetylene) is filled, is placed between two vitreosil substrates. After the liquid crystal in the isotropic arrangement is illuminated by polarized UV light, the light-induced liquid-crystal molecule arranges orientationally. Results of numerical simulation show that the structure of the forbidden band of the 2-D triangular photonic crystal can be tuned by manipulating the photoinduced direction of the nematic LC molecules by the aid of external light field. Since the selective transmission of TM- and TE- polarized modes in the waveguide can be controlled by the adjustable photonic crystal, it can be used to manufacture the alloptical switch in photonic integrated circuits.