Conformational analysis of poly(ethylene terephthalate) (PET) and poly (ethyl ene-2,6-naphthalate) (PEN) has been carried out by the refined rotational isomeric state (RIS) scheme coupled with ab initio molecular orbital (MO) calculations for a model compound, ethylene glycol dibenzoate (EGDB). H-1 and C-13 NMR experiments for unlabeled and C-13-labeled EGDBs yielded bond conformations of the central O-CH2-CH2-O bond sequence. The MO calculations satisfactorily reproduced not only the experimental bond conformations but also the dipole moment and molar Kerr constant observed from EGDB. The aromatic ring and ester group of EGDB render its intramolecular interactions more complicated than those of poly(ethylene oxide) with the same O-CH2-CH2-O bonds; C=O center dot center dot center dot H-C and C-O center dot center dot center dot H-C close contacts and dipole-dipole interactions are simultaneously formed in EGDB. The characteristic ratio of PET, derived from the refined RIS calculations using the MO energies for EGDB, exactly agree with those determined from small angle neutron scattering experiments for the melt. Thermodynamic quantities on PET and PEN, evaluated from the RIS calculations, have well characterized melting and crystallization behaviors of these polyesters.