A method of reparation of prussian blue (PB) from a single ferricyanide solution and the mechanism of its formation are discussed : In a weak acidic medium (pH 3.6) of KH2PO4 (0.1 M)-HCl, a thin film of PB is deposited on a gold electrode surface by using cyclic voltammetry between -0.2 and + 0.6 V. It is confirmed that the coincident effect of the solution acidity on the PB preparation is exhibited by using either cathodical polarization or cyclic voltammetry. The UV-vis absorption spectra from the various acidic solutions indicate that the absorption curve of pH at 3.6 is different from those of other pH values. Under the same conditions, PB cannot be deposited on a gold electrode surface from the ferrocyanide solution by using cyclic voltammetry and cathodical polarization. in addition, a kind of blue precipitate is gradually produced in the ferricyanide solution during standing after one day, and this is verified to be PB by the two pairs of its characteristic redox peaks at about +0.2 and +0.8 V vs. Ag/AgCl, which are obtained by immobilizing the blue precipitate mechanically on a gold electrode surface and recording the cyclic voltammogram while no similar precipitate occurs in the ferrocyanide solution during its standing course. Furthermore the Fe2+ cannot be oxidized to Fe3+ on a gold electrode even when the positive potential reaches to +1.0 V, while Fe3+ can be easily reduced to Fe2+ at -0.2 V. It is concluded that the PB formation from the ferricyanide solution lies in the dissociation of ferricyanide ions.