The configuration of cationic polymer chains chemically grafted on a substrate polymer surface was studied on the basis of the zeta potential. We employed a poly(ethylene terephthalate) film and (dimethylamino)ethyl methacrylate as the substrate polymer and the cationic monomer, respectively. Graft polymer chains having different lengths but a constant number density were covalently immobilized on the film surface by graft polymerization of the monomer using the simultaneous UV irradiation method. It is found that there appears mostly a maximum in zeta potential and a minimum in contact angle when they are plotted against the molecular weight of the graft chain. This suggests that when the graft chains are short in length, they probably do not interfere with each other and are in a quasi-soluble state, freely extending into the outer aqueous phase, whereas, lateral interaction among the graft chains may occur with the increasing length of graft chains, resulting in interweaving of graft chains or partial formation of chain cluster because of their poor solubility in water. The water solubility of the cationic polymer greatly depended on the molecular weight of the graft chain, the pH value of the aqueous medium, and the ion complexation. Based on these findings, we propose a configuration model for the graft chains on the surface.