A dual-band (UHF: 900-930 MHz, ISM: 2.35-2.5 GHz for North America Regulations)circularly polarized aperture coupled microstrip RFID reader antenna with separated transmitting (Tx) and receiving (Rx) ports has been designed, fabricated, and measured. The maximum measured LHCP antenna gain is 8.2 dBic at 910 MHz (UHF) and RHCP gain is 8.8 dBic at 2.45 GHz (ISM). The isolations between the two ports are about 10 dB and 33 dB, at 910 MHz and 2.45 GHz, respectively. The measured axial ratios are less than 2dB in the UHF band (870-950 MHz) and 3 dB in the ISM band (2.35-2.5 GHz). The proposed antenna is able to meet all the requirements for RFID reader applications.