We have recognized that a vacuum-deposited silicon film shows very attractive Li insertion/extraction reaction performance, but the reaction rate in view of Li diffusion in the silicon film has not been determined yet. In this study, we performed analysis of the reaction rate by the means of potential-step chronoamperometry (PSCA) method. Samples of the silicon films of several different thicknesses ranging from 500 to 14000Å were prepared by depositing silicon in vacuum on an etched Ni foil. The current–time curves were recorded under PSCA condition and offered to evaluate the apparent diffusion coefficient D app of Li in the silicon films. The obtained D app values were as low as 10−10 to 10−13 cm2 s−1, which were dependent on the film thickness.