In the present study, the effects of the anodic electrochromic layer employed in the all-solid-state ECD in the structure of ITO/NiO/Ta2O5/WO3/ITO/glass on the optical and electrochromic performances of the device are investigated. For this purposes, the nickel oxide films with various thicknesses were deposited onto glass and ITO-coated glass substrates by RF magnetron sputtering technique using a NiO target in pure Ar atmosphere at room temperature. The optical, structural and electrochromic properties of the NiO films with various thicknesses (140 nm, 230 nm, 310 nm and 480 nm) were investigated. The optical transmission of the films decreased as the thickness of the films increased. The X-ray diffraction patterns showed that the films were polycrystalline phase and the peak density increased with the film thickness. The optical band gap was found to decrease from 3.71 to 3.50 eV with increasing film thickness. The root-mean-square (rms) surface roughnesses of the films increased (from 1.43 nm to 2.94 nm) with increased film thickness. Coloration efficiencies and optical modulations of the NiO films were obtained for a wide spectral range. The amount of inserted/extracted charges into/from the films during bleaching/coloring were obtained as a function of the film thickness. The highest coloration efficiency (40.7 cm2/C) and optical modulation (27.8%) at a wavelength of 550 nm were obtained for the NiO film with a thickness of 480 nm. Finally, all-solid-state electrochromic devices were fabricated by using the thinnest and thickest NiO films with a configuration of ITO/NiO/Ta2O5/WO3/ITO/glass. The ECD-1 device with a thinner NiO layer showed a high optical modulation (36.4%). On the other hand, the ECD-2 with a thicker NiO layer showed a very low optical modulation (1.5%) at 550 nm. Our results indicated that the thicker NiO film exhibited better electrochromic properties in the half-cell configuration, while the thinner one showed higher optical modulation in electrochromic device fabrication with a complete-cell configuration. Therefore, the effect of the thickness of a NiO layer on the electrochromic performance of both the half-cell and complete cell configuration electrochromic coatings were investigated in detail.