The plane wave expansion method is used to calculate the band gap of two-dimensional photonic crystal in the E and H polarization, with Taking into account the effects of filling ratio, lattice structure, and relative dielectric constant ratio on the maximum absolute band gap. The results show that either square or triangular array arrangement, it is easier for the dielectric cylinder type photonic crystal to form TM-polarized mode band gap; while, in the air-hole photonic crystal it is much easier to form TE-polarized-mode band gap. For a given filling ratio, with the increase of relative dielectric constant, the maximum gap width does not always increase, but it is peaked. Similarly, for a given relative dielectric constant, with the increase of filling ratio, it is also peaked for the maximum gap width. Moreover, no matter the erystal is air hole type or dielectric cylinder type in structure, the triangular lattice structure is much easier to form bands gap.