The effect of line defects on photonic band gap was investigated for the two dimensional photonic crystal consisting of square lattices of dielectric cylinder by means of plane wave expansion method. The result shows that both the radius and the width of defect layer in photonic crystal have an obvious effect on photonic band gap. The first band gap width of the TE wave was a maximum with the increase of the radius of the defect cylinders. The centre of the band gap shifts down and the numbers of band gap are changed well with increase of the radius of the defect cylinders. In addition, the band gap width is reduced as the difference between the width of defect layer and that of normal layer increases.