The fabrication of polyester core-shell toner by emulsion aggregation method was investigated to realize better balance of low-energy fusing and good storage stability. In core-shell toners which use high Tg shell resin and low Tg core resin, the storage stability can be significantly worse than expected from shell Tg. It was thought that the core moiety diffused into the shell by interfacial polymer diffusion in the heating coalescing process. From this viewpoint, the control of the interfacial diffusion (interfacial thickness: λ) between the core and shell was examined based on the χ parameter of the polyester resin combinations which was calculated by monomer composition. After examinations of several polyester combinations based on calculated χ parameters, it was found that 1,2-propanediol (1,2PD)/isophthalic acid (IPA) polyester and 1,2PD/alkenyl succinic anhydride (AS) polyester showed a strong phase separation. The ellipsometry measurement revealed that the interfacial thickness of this combination was only 10nm, which was too small for sufficient adhesion between the core and shell. Therefore larger interfacial thicknesses were tested by incrementally decreasing the χ parameter by changing monomer composition. A strong relationship between actual λ and the χ parameter was found. The λ could be controlled at 80nm by using a resin combination based on 1,2PD/IPA and 1,2PD/IPA/AS, and a core-shell toner was produced using this combination of polyester resins. The adhesion between core and shell was found to be sufficiently strong, and the relationship between low temperature fusing and storage stability became significantly better compared with the toner having miscible core and shell.