The influence of applied voltage on cationic electropainting was investigated and the deposits were observed by atomic force microscope (AFM), using an acrylic resin with a backbone of methyl methacrylate as a model of a practical cationic resin. The film formation mechanism including the initiation process are discussed. The bath solution which dispersed cationic polymer particles was stable without precipitation. At applied voltage lower than 5 V. the particles deposited and formed a very thin film (nanometer size). Moreover, at applied voltages higher than 5 V. the deposit weight increased abruptly and a visible film was formed. The deposit weight was proportional to coulombs passed. The AFM image of the film showed the porous structure where the particles of 50 to 100 nm in diameter coagulated. Because film formation could be observed by AFM even during the induction period, it is concluded that the film can be deposited on the electrode at much lower pH than 12. The site of growth was the interface of the deposited layer and bulk solution. On the basis of the results. we propose a generalized mechanism of the film formation including the initiation process.