Band structures of silicon photonic crystal (PC) with different lattices and shapes of air holes at telecom wavelengths were investigated by plane-wave expansion method, and the related physical models were proposed. Calculated results demonstrate that photonic band gap (PBG) can be effectively manipulated by photon confinement effect and lattice symmetry effect. With the increase of filling fraction, the ability with which photons are confined by PC is enhanced, PBG is opened and the central frequency undergoes a blue-shift. PBG is enlarged as the lattice symmetry increases. Shape and rotation of lattice element are also studied. Band gap with the rotation angle which follows periodicity and symmetry indicates its anisotropy. The optimal cavity structures for different lattices are also found.