In order to design photonic crystals with complete band gaps, a topology optimization algorithm is proposed based on finite element analysis and bi-directional evolutionary structural optimization method. The photonic crystals are assumed to be periodically composed of two materials with different electromagnetic property. By introducing discrete design variables and calculating the sensitivity of each element, the BESO algorithm gradually re-distributes the dielectric materials within the unit cell until the photonic crystal have a complete band gap between specified photonic bands. The proposed optimization algorithm is efficient and some innovative designs have been obtained.