A Prussian blue analog (PBA), copper hexacyanoferrate (Cu-PBA) has been synthesized as an insoluble aggregated solid of nanoparticles (NPs). The Cu-PBA NPs bore negative surface-charge via their surface modification using [FeII(CN)6]4- and were stably dispersed into water from the insoluble solid. Spin-coated thin films consisting of the Cu-PBA NPs on ITO glass substrates showed reversible electrochromism of red (reduced form) and yellow (oxidized form). The spectroscopic investigation revealed that the reduced form of Cu-PBA was a mixed valence form included Cu(I).