Copolyesters composed of poly(ethylene 2,6-naphthalate) and poly(ethylene 9,10-dihydrophenanthrate) (PEN/PEHP copolymers) with different copolymer compositions were synthesized and their thermal properties and fine structures were investigated using thermal analysis and X-ray diffraction methods. T(g)s Of the copolymers increased with increasing molar fraction of the PEHP component; on the other hand, their T(m)s decreased slightly and their crystallinity also decreased. The crystallites in the copolymers were assigned to the alpha- or beta-polymorphic forms of PEN. In particular, annealed samples containing 5% and 10% PEHP exhibited X-ray diffraction patterns showing the coexistence of the alpha- and beta-forms. In the case of samples containing 20% PEHP the crystallites were of the beta-form. Thus, the crystalites formed in the copolymers are composed of PEN sequences, while PEHP sequences remain in the amorphous regions. PEHP sequences interfere with the crystallization of PEN/PEHP copolymers and have a strong influence on the chain packing mode of the unit cell in the copolymers.