The proposed design is a CMOS-based, time-to-digital, on-chip and lost-cost temperature sensor. Without relying on any bipolar transistor or analog-to-digital converter, the required chip area is relatively small and the power consumption is low. Its simple structure allows it to be freely embedded onto any CMOS IC chip. The sensor generates a digital code proportional to temperature by time-to-digital method. The code with a lookup table can be directly used to indicate current thermal status. The test chip fabricated in the UMC CMOS 90nm 1P9M process has a extremely small area of 0.01004mm2. The effective resolution is around ±0.14℃ without calibration and exactly 0.1℃ plus perfect linearity with calibration.