To investigate the microstructure of nickel oxide electrochromic thin films prepared by reactive DC magnetron sputtering, cross-sectional observations of sputtered nickel oxide films have been performed using a high resolution electron microscope. High resolution images were observed for as-deposited and bleached samples. These images show that the surface morphology of these samples is quite different. The bleached sample consists of needle-like microcrystallites, while the as-deposited sample consists of columnar crystals and has a smooth surface. Diffraction patterns indicate that crystallized NiO is the dominant structure in both samples. The fact that no trace of Ni(OH)(2) was observed implies that the boundary and the surface of NiO microcrystallites play important roles in the electrochromic reaction.