The elastic modulus (E(l)) of the crystalline regions in the direction parallel to the chain axis was measured for p-hydroxybenzoic acid (HBA)/poly(ethylene terephthalate) (PET) copolymers by X-ray diffraction. The E(l) values are 59 and 105 GPa for HBA/PET 60/40 and 80/20 copolymers, respectively, which are considered to reflect the linearity of the skeletal structure of the chain molecule. The elastic modulus (E(t)) of the crystalline regions in the direction perpendicular to the chain axis was 2.1 GPa for both copolymers, which was much smaller than that of polyethylene (similar to 4 GPa, where only van der Waals forces act). Though pi-electron interaction could be expected, loose and/or disordered lateral packing of the chains leads to the small E(l) values of these copolymers.