Elastic modulus of the crystalline regions (crystal modulus) of poly(glycolic acid) (PGA) in the directions parallel (E-l) and perpendicular (E-t) to the chain axis was measured by X-ray diffraction. The E-l value of PGA was 104 GPa at room temperature. This relatively low E-l value suggests that the PGA skeleton is not fully extended as reported before but contracted. The E-t value of PGA was obtained as 7 GPa, which resembles that (7 GPa) of poly(vinyl alcohol) based on intermolecular hydrogen bonds. This reveals strong intermolecular interactions act between PGA molecules in the crystal lattice. Both low E-l value and high E-t value are considered to contribute to the extraordinarily high melting point (223 degrees C) of PGA among aliphatic polyesters. Temperature dependence (13-400 K) of both E-l and E-t of PGA was also investigated. Abrupt changes around 200 K for both directions were considered to be caused by molecular motion in the crystalline regions of PGA.