We have studied the optical transmission of one-dimensional photonic crystals with a mirror symmetry. One or more complete transmission peaks exist in the forbidden gap. A simple recursion relation is derived by means of the mathematical induction to calculate the frequencies of the complete transmission peaks. In the center there is always a transmission peak due to the mirror symmetry. Other peaks are dependent on positions of the defects and the refractive index. If the structure is chosen properly, a wider transmission band can be obtained.