A synthesis method for symmetric and asymmetric dual-band microwave filters is presented. The proposed method is based on the frequency transformation technique, and the poles and zeros of a desired dual-band filter can be determined by the low-pass prototype filter. To simplify the filter design, an analytical method based on iterations is introduced to eliminate the redundant transmission zeros. The remaining transmission zeros can be rearranged to improve the frequency selectivity. To validate this method, a microstrip open-loop symmetric dual-band filter has been designed and fabricated. The measured results are in good agreement with the simulated ones.