In this paper we present a temperature sensor based on a ring oscillator. The ring oscillator uses the CMOS thyristor delay element and has an extremely low power consumption of about 47nW at room temperature with a supply of 0.5V. Low power operation is achieved by eliminating the use of power hungry analog to digital converters (ADCs) at the sensor output. As shown the frequency increases linearly with temperature. The error in temperature sensing is around -1.8°C / +1°C with a resolution of 0.3°C. Simulation is carried out with Chartered Semiconductor's 0.18μm technology. Owing to the extremely low power consumption, integration with a radio-frequency identification (RFID) tag is also possible.