In order to meet the application requirement of passive RFID, a low power CMOS temperature sensor was designed in SMIC 0.18 μm CMOS process. The sensor firstly converted the temperature signal into a voltage one through double-transistor circuits and then into a current signal. Afterwards, the signal was converted into a frequency one by a ring oscillating circuit. Eventually the signal was output in the form of a binary number by a counter circuit. Simulation results demonstrate that the sensor achieves good linearity and accuracy in the range of -20-100 ℃, and only consumes power of 1.05 μW, which can be potentially applied in the field of RFID.