In this paper, an low power CMOS temperature sensor for implantable applications is implemented in a 0.18 μm CMOS process. Sensors for implantable devices must have sub-μW power consumption to avoid tissue overheating. Thus, this temperature sensor employs subthreshold MOS transistors as the sensing element to reduce power consumption and enable minimum supply voltage. Temperature is converted to frequency using a pW voltage reference. The sensor core operates under 0.5 V, with total power dissipation of 290 nW and achieves temperature measurement of changes smaller than 0.5 °C for a range from 0 ° C to 50 °C.