A dual frequency circularly polarized antenna is proposed for UHF (950MHz) and microwave (2.45GHz) RFID reader applications. By properly combined circularly polarized square ring slot structure into well known circularly polarized truncated square patch antenna, a dual-frequency circularly polarized microstrip antenna is presented. The proposed antenna is located on the finite dielectric substrate. In the numerical analysis, the electromagnetic simulator WIPL-D based on the Method of Moment is used.