A process for preparing a polyester comprising the steps of blending (A) polyethylene terephthalate before solid phase polymerization having an intrinsic viscosity of 0.3 to 0.8 dl/g in an amount of 40 to 99% by weight, with (B) polyethylene naphthalate before solid phase polymerization having an intrinsic viscosity of 0.3 to 0.7 dl/g in an amount of 1 to 60% by weight, and subjecting the blend to solid phase polymerization. The process of the invention can provide a polyester having excellent gas barrier properties, transparency and heat resistance and hardly generating acetaldehyde.