This paper presents a low-cost low-power humidity sensor for applications of ultra-high frequency radio frequency identification sensing tag. The humidity sensor element, based on standard SMIC 0.35 μm complementary metal-oxide-semiconductor technology, utilizes polyimide as sensing material and fabricates the interdigitated electrodes in top metal layer without any further post-processing. The humidity sensor interface, based on phase-locked loop theory, employs fully-digital architecture and achieves direct capacitance-to-digital conversion, which allows the supply voltage to be close to threshold voltage. The measurements at 25 ℃ show that the proposed humidity sensor achieves a sensitivity of 36.5 fF%RH, maximum hysteresis error of 7%, response time of 20 ms, and 2.1 μW power dissipation at 0.6 V supply voltage.