The band gaps of the two-dimensional phononic crystal composed of square array of cylinders in air are analyzed by the plane-wave expansion (PWE) method. The numerical result shows that the band gaps are independent of the physical parameter of the scatter if the acoustic impedance of the scatter is much greater than that of the matrix. So the band gaps are determined by the radius of the cylinders and the lattice constant. It doesn't matter whether the cylinders are hollow or not.