This work presents an ultra-low power CMOS temperature sensor in 0.13渭m standard CMOS process with an area of 0.0014mm2 and a power consumption of 0.15渭W. CMOS transistors operating in subthreshold region generate the PTAT current, and a relaxation oscillator converts the current into frequency, which is digitalized by a following counter. The obtained oscillation frequency is nearly linear to the temperature, and a two-point calibration is applied to reduce the temperature spread in the range -0.27掳C to 0.3366掳C.