An electrochromic thin film of amorphous Mo0.5V0.5O2.75 was fabricated on an ITO glass substrate by the spin-coating technique using a solution of a mixed peroxo-polyacid based on Mo and V (denoted as Mo/V-HPA with the mole ratio of Mo/V=1). The Li insertion dynamics of the film in contact with a Li+ doped solid polymer electrolyte was investigated by the chrono-amperometric method. It was confirmed that the insertion rate is controlled by the Li diffusion process with the chemical diffusion coefficient about 1x10(-11) cm(2)/s (25 degrees C).