Nano-porous nickel oxide (NiO) film is coated on the indium tin oxide glass using nickel acetate solution containing block copolymer as the surfactant by the sol-gel method with a spin coating technique. The block copolymer assists the porosity of NiO thin film, which improves the ion intercalation and deintercalation. The electrochromic properties of NiO thin films with different thickness are investigated in an aqueous alkaline electrolyte (0.1 M KOH) by means of cyclic voltammetry, chronoamperometry and transmittance measurements. The nano-porous structure of NiO thin film gives good reaction kinetics with high diffusion coefficient of 1.79 x 10(-9) cm (2) s(-1). In the case of NiO thin film with thickness of 170 nm, the electrochromic properties are observed with the coloration efficiency of 129.73 cm(2) C-1, coloration/bleaching time of 7.95/3.33 s and transmittance modulation of 28% at a wavelength of 550 nm. The thinner NiO film is, the better diffusion coefficient and coloring change time are. However, the transmittance modulation and coloration efficiency of the device are better as NiO film is thicker. Therefore, it is important to optimize the electrochromic device parameters depending on application area because they are trade-off relation. (C) 2017 Elsevier B.V. All rights reserved.