An investigation was undertaken to develop a new class of water-borne polyesters having excellent dispersibility without using emulsifiers. Grafting reactions of hydrophilic acrylics to polyesters and the relationships between reaction conditions and the dispersion diameters are discussed. Aqueous dispersions of particles having small diameters were obtained by use of an unsaturated polyester. Particle diameters were related to the grafting efficiency that could be enhanced by copolymerization of reactive monomers with the unsaturated bonds of polyesters. The relationships between particle diameters and the kind of acrylic monomers were described by the copolymerization reactivity ratio of the unsaturated polyester and acrylic monomers. Also, the influences of the solvency of the reaction medium and the influence of concentration of unsaturated bonds of the polyesters in the reaction medium were discussed to explain the low reactivity between the unsaturated bonds and acrylic monomers.