Polyaniline (PANI) was prepared in emeraldine via chemical oxidation method using CuSO4 as initiating agent. The chemical characterizations were made using UV-vis (ultraviolet-visible spectrophotometry), FT-IR (Fourier transform spectroscopy), TG-DTA (thermo-gravimetric/differential thermal analysis), ESI-MS (electrospray ionization mass spectrometry), XRD (X-ray diffraction), SEM (scanning electron microscopy) techniques. The results revealed the formation of homogeneous, crystalline PANI with sub-micron size particles. The PANI thin film of 0.5 µm thickness has been fabricated using spin coating technique. The resulting PANI film was exposed to controlled humid condition and change in resistance has been recorded. The resistance was continuously decreased from 13.5 to 3.75 MΩ with a linear change in humidity ranging from 3 to 95%. The result was reproducible and checked by repeating observation.