Nickel oxide (NiO) thin films were deposited onto glass substrates and ITO-coated glass substrates by RF magnetron sputtering using a NiO target in pure argon (Ar) atmosphere at room temperature. The chamber working pressure was kept at 15 mTorr and deposition power was 75 W during the deposition processes. The films were annealed for 2 h between 100 degrees C and 400 degrees C in air ambient. The optical, structural and electrochromic properties were investigated by optical spectrophotometry, X-ray diffraction, atomic force microscopy and electrochemical measurements of the samples, respectively. Effects of the heat treatment on the optical constants of the films were analyzed in detail. Coloration efficiencies and optical modulations of the films were estimated for a wide spectral range. The amount of inserted/extracted and residual charges of the films during the bleaching/coloring process was calculated as a function of annealing temperature. Finally, a custom design all solid-state electrochromic device was fabricated using the NiO film as an anodic layer developed on this study.