A novel ultra low power high precision temperature sensor IC is presented. The temperature sensor consists of a temperature dependent oscillator, and the output of the oscillator is compared to a temperature independent frequency signal to generate the digital output. A novel ring oscillator has been used for the temperature dependent oscillator, to reduce the power consumption and to improve the measurement accuracy. The temperature sensor IC is realized in the standard 0.18 CMOS process, and the circuit die area is only 0.08 mm2. Simulation results show that the temperature measurement accuracy is 0.1 degree after calibration. The current consumption of the sensor IC is only 0.2μA .