Transmission characteristics of electromagnetic wave in 1D rectangle photonic crystal are obtained with restrictions condition of electromagnetic wave and the characteristics of mode are studied.Transmission characteristics of electromagnetic wave are calculated by the dispersion method.The new characteristics of electromagnetic wave in 1D rectangle photonic crystal are obtained.The bandgap of 1D rectangle photonic crystal is determined by quantum number of mode.The center and width of the bandgap are determined by the quantum number of mode and the side length of rectangle.