Six copolymers containing poly(tetramethylene 2,6-naphthalene dicarboxylate)(PTMN) in the hard segment of polyetheresters were synthesized. Thermal properties of polyetheresters were investigated by DSC and TG/DTA instruments. Crystalline properties of them were calculated by the DSC data. Incorporation of dimethyl 2,6-naphthalenedicarboxylate(NDC) in the polyetheresters resulted in a gradual increase in the glass transition temperature. However, the trends of the molting point thermal decomposition temperature, and stability of crystalline structure turned in the opposite directions after the point of 44.4 mole% of NDC. Heat resistance of polyetherester could not be obtained over the 11.1 mole% of NDC.