The band gaps of a two-dimensional phononic crystal are studied by using the plane-wave expansion method and the supercell calculations. The two-dimensional phononic crystal is formed by square-shape arrangement of 4340 steel inserted into epoxy resin. The band gaps of different structures are calculated such as defect-free, 45 crystal defect states, 90 crystal defect states, 135 crystal defect states and 180 crystal defect states. It is found that the bandwidth of defect state is about 31 times larger than that of the defect-free crystal; with F=0.10.9 the band gaps occur in different defect structures and increase with defect state angle increasing. In addition, the influences of filling fraction on the band gap number and the relative width of the minimum band gap are also discussed.