We present an ultra-low power temperature sensor embedded in the passive RFID tag using the TSMC 1P6M 0.18 μm m standard CMOS process. Substrate parasitic NPN bipolar pair is exploited to generate the temperature dependent current signals for thermal sensing. A time-domain readout scheme which has high immunity to the on-chip resistor, capacitor and clock frequency process-voltage-temperature (PVT) spreads is further proposed. Measurement results of the embedded sensor within the tag system shows a sensing accuracy of pm 1.5̂C (3σ) from-30̂C to 60̂C after one-point calibration at 20̂C, with a sensing resolution of 0.3̂C and a sampling rate of 68 samples per second. The embedded sensor draws 0.35 μA from a 1 V supply at room temperature and occupies a chip area of 0.14 mm2. © 2013 IEEE.