Thickness-dependent microstructural and electrochromic properties of sputter-deposited Ni oxide films were investigated as a function of growth time using x-ray diffraction, in situ transmittance measurements, and x-ray photoelectron spectroscopy. By increasing the thickness of the Ni oxide, the transmittance or optical differences during coloring/bleaching process were increased. However, thick Ni oxides showed a lower maximum bleached transmittance and coloration efficiency and a larger response time. Crystallinity developed with growth time, mainly due to the plasma heating effect. The evolution of crystallinity with the growth time resulted in the electrochromic inactive Ni oxide components even after several tens of potential cyclings, leading to a decreased maximum bleached transmittance and lower coloration efficiency.