In this paper, a fiber optic Fabry–Perot (FP) for relative humidity (RH) sensing is presented. The proposed FP cavity is constructed by splicing a 50-mm length of no-core fiber (NCF) in a single mode fiber. Then, the end side of the NCF is coated with a polyvinyl alcohol (PVA) thin film membrane with different thicknesses (1, 2, 3, 4 µm respectively) to work as a mirror. The fringes pattern of the FP undergoes a spectral shift owing to the alteration in the PVA refractive index with the ambient RH alternative. The highest obtainable sensitivity was observed at thickness of 3 µm, which is about 0.866 nm/RH%. After that, the diameter of the NCF is tuned from 125 to 65 µm using hydrofluoric acid (HF40%) to maximize the evanescent field and thus improved sensitivity to about 0.908 nm/RH% at the diameter of 95 µm for 30% to 90% RH range. The sensor shows good stability, and easy fabricated.