Energy efficient and low area temperature sensors are critical for constantly monitoring the silicon temperature in high density integrated circuits. In this paper, a 0.2nJ/sample ring oscillator based temperature sensor is designed and fabricated in a 65nm CMOS technology. The sensor achieves a maximum inaccuracy of ±3°C after 2-point calibration and a resolution of 0.3°C. Furthermore, a model is proposed to accurately predict the effect of the oscillator phase noise on the error of the sensor. The chip occupies only 0.01mm2.