This paper presents a modulator and a demodulator designed for UHF RFID readers. The proposed circuits are designed and implemented in a 0.18 μm CMOS technology and measurement shows that the modulator has an input 1 dB compression point of 14 dBm, insertion loss of 1.17 dB, and isolation of 33 dB. The demodulator has an input 1 dB compression point of 5 dBm, noise figure of 9 dB, and voltage conversion gain of 4.9 dB, which is negative in the conventional structure. Neither dissipates DC power.