This paper presents a power-efficient temperature-to-digital convertor using a leakage-based ring oscillator. The reversely-biased transistors are added between the supply rails and the core circuit to limit power consumption and enhance temperature-to-current conversion gain. A design of temperature-insensitive sampling clock signal results in better conversion linearity/accuracy. The design was fabricated in a 180-nm CMOS process with an active chip area of 0.04 mm2. The design can achieve a resolution of 43 mK at 20 °C in 100 measurements and a temperature inaccuracy of -1.6/+2.1 °C over a temperature range of 20-80 °C. At a 0.55 V supply, the power consumption of the whole system is 950 pW, the conversion time is 40.8 ms, and the resulted energy efficiency is 39 pJ/conversion.