In this paper, we propose a high-resolution all-digital complementary metal–oxide semiconductor (CMOS) temperature sensor with a ring oscillator for temperature sensing and power supply immunity, a time amplifier for process variation compensation, and serial digital output for low area consumption. Because of the linearity of its high output cycle–temperature, the sensor has excellent measurement accuracy; the architecture of the sensor effectively decreased power supply sensitivity, and the parallel-to-serial converter considerably reduced area consumption. In addition, the high resolution is determined by the time amplifier with external digital codes. The determination of resolution using the time amplifier and the process variation compensation were accomplished simultaneously. The temperature sensor was fabricated using 0.18-μm standard CMOS technology, and the core circuit occupies an area of 0.001 mm2. The experimental results indicated that the energy per conversion rate was only 10 nJ at a supply voltage of 1.8 V; the conversion rate was 15–30 k samples/s, and the error in temperature sensing ranged from −1.58 to +1.6 °C with a resolution higher than 0.1 °C after one-point calibration in the −40 to +130 °C range. With these advantages, the temperature sensor is suitable for application in large integrated circuit (IC) systems and three-dimensional ICs.