In full color laser printers and copiers, the performance requirements include high printing speed, fine image quality, and environmental friendliness. Specially designed toners are needed to achieve these requirements. To develop the toner for low-energy fusing, hot-offset resistance, and good storage stability, chemically prepared polyester toner was developed (core-shell typed toner) by emulsion aggregation method. Polyester emulsion is usually made by phase inversion emulsification with organic solvent. Unfortunately higher molecular weight polyesters, which provide resistance against storage stability and hot-offset, have limited solubility in organic solvent. To solve these problems, some cross-linking techniques like urethane reaction and oxazoline-polymer reaction have been developed. But these techniques resulted in disappearance of carboxyl groups which play a role in the dispersion stability of emulsion, so emulsions with small and narrow particle size distribution could not be made. It is a challenge to design resins that have both larger molecular weight and dispersion stability.In this study, to support larger molecular weight and dispersion stability of polyester emulsion at the same time, we introduced seed polymerization which allows radical polymerization inside emulsion without disappearance of carboxyl groups. In addition, it is found that seed polymerization could control the thermal properties of polyester emulsion like storage elastic modulus and glass-transition temperature. With this method, we could design polyester chemically prepared toners having a wider range of thermal properties.