A 900MHz Class E CMOS Power Amplifier (PA) integrated in an UHF RFID reader using a 0.18 um CMOS technology has been proposed in this paper. The Class E PA employs a two-stage single-ended cascode topology. The single-ended architecture is used to occupy a relatively small area, to lower the cost and to reduce the power consumption of the whole RFID reader system. Small area, low cost and low power consumption are especially important to a handheld UHF RFID reader system. The cascode structure can ease the voltage stress over the devices in the power stage of the PA. Prototypes, proposed in this paper, demonstrate a 40% PAE (Power Added Efficiency) while delivering 20dBm output power at 900MHz.