The fabrication of a nanowire WO3 humidity sensor integrated with an inverting amplifier circuit and a micro-heater on a chip using the commercial 0.35 μm complementary metal oxide semiconductor (CMOS) process and a post-process have been implemented. The humidity sensor is composed of a sensing resistor and a humidity sensing film. Tungsten trioxide prepared by a sol–gel method is adopted as the humidity sensing film. The fabrication of the humidity sensor requires a post-process to etch the sacrificial layers and to expose the sensing resistor, and then the humidity sensing film is coated over the sensing resistor. The humidity sensor, which is a resistive type, changes the resistance when the sensing film adsorbs or desorbs water vapor. An inverting amplifier circuit is utilized to convert the resistance of the humidity sensor into the voltage output. The micro-heater is utilized to provide a super-ambient working temperature to the humidity sensor, which can avoid the humidity sensor to generate the signal drift. Experimental results show that the sensitivity of the humidity sensor is about 4.5 mV/% RH at 60 °C.