Coloring mechanism of tris-1, 10-phenanthroline nickel complex was evaluated with spectro-electrochemical methods. Tris-1, 10-phenanthroline nickel complex colored violet by applying cathodic voltage in the tetrabutylammonium perchlorate/DMF solution. The coloring rate of tris-1, 10-phenanthroline nickel complex was determined by the diffusion of this complex near the cathode. On the other hand, 1, 10-phenanthroline was also revealed to exhibit electrochromic phenomena similar to tris-1, 10-phenanthroline nickel complex. Supporting electrolyte concentration dependence was found in the electrochromism of 1, 10-phenanthroline. However, this tendency was not observed in the electrochromism of tris-1, 10-phenanthroline nickel complex. These suggest that 1, 10-phenanthroline colors as the result of interaction with tetrabutylammonium perchlorate. 1, 10-Phenanthroline was revealed not to color in itself but in the interaction with supporting electrolyte or metal ions under reduction. Namely, in tris-1, 10-phenanthroline nickel complex system, 1, 10-phenanthroline interacted with nickel ion stronger than with tetrabutylammonium perchlorate. Therefore, tris-1, 10-phenanthroline nickel complex showed higher electrochromic efficiency at lower applied voltage than 1, 10-phenanthroline system.