A fully on-chip temperature, process, and voltage sensor includes a voltage sensor, a process sensor and a temperature sensor. The temperature sensor includes a bias current generator, a ring oscillator, a fixed pulse generator, an AND gate, and a first counter. The bias current generator generates an output current related to temperature according to the operating voltage of chip. The ring oscillator generates an oscillation signal according to the output current. The fixed pulse generator generates a fixed pulse signal. The AND gate is connected to the ring oscillator and the fixed pulse generator for performing a logic AND operation on the oscillation signal and the fixed pulse signal, and generating a temperature sensor signal.