Copolyesters containing rigid segments (naphthalene and terephthalene) and flexible segments (aliphatic diol) structure were synthesized from DMN/DMT/EG (2,6-dimethyl naphthalate/1,4-dimethyl terephthalate/ethylene glycol) ternary monomers with various mole ratios. Copolyesters having intrinsic Viscosities of 0.52-0.65 dL/g were obtained by melt polycondensation in the presence of metallic catalysts. The effect of reaction temperature and time on the formation of the copolyesters was investigated to obtain an optimum condition for copolyester manufacturing. The optimum condition for PNT (polyethylene naphthalate terephthalate) copolyester manufacturing is the transesterification under nitrogen atmosphere for 4 h at a temperature of 185 +/- 2 degrees C followed by polymerization under 2 mm Hg for 2 h at a temperature of 280 degrees C. Most copolyesters have better solubilities than poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET) in various solvents. The effect of the starting mole ratio of DMN, DMT, and EG on the thermal properties of the resulted copolyesters was also investigated using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Glass transition temperatures of copolyesters were in the range of 70.7-115.2 degrees C, and 10% weight loss in nitrogen were all above 426 degrees C.