This paper proposed a low-power smart temperature sensor. The sensor consisted of a PTAT circuit, and a ring oscillator. The current of the PTAT circuit was used to drive the ring oscillator which generates a temperature related signal. The sensor was implemented by the TSMC CMOS 0.35 μm 2P4M digital process. The core area is only 1105.59 μm2. The power consumption is about 159.15 nW. The linearity between the output frequency and temperature is marked by R-square rule. The value of the linearity is 0.991 during the temperature range. The proposed sensor required only one supply voltage. The core area was small. Therefore, the sensor was suitable for embedded in any circuit that required temperature monitoring.