In this paper, the photonic band gap (PBG) properties of two dimensional (2D) square lattice photonic crystal structures composed of rectangular cells were studied. The effect of refractive index, rectangles length and the ratio of width to length of the rectangles on the PBG properties of the structure with different configurations was investigated. It is found that the density of gaps in both modes (transverse electric (TE) and transverse magnetic (TM)) is high for structure composed of rectangular dielectric rods in air, while the density of the gaps is very low for structure composed of rectangular air pores in dielectric material.