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Elastic moduli of the crystalline regions (crystal modulus) of poly(lactic acid)s (PLA), poly ( l -lactic acid) (PLLA), poly( d -lactic acid) (PDLA) and their stereocomplex (scPLA) in the directions parallel ( E l ) and perpendicular ( E t ) to the chain axis were measured by X-ray diffraction. The E l value of PLLA and PDLA coincided as 14 GPa, whereas the E l value of scPLA was 20 GPa at room temperature. These relatively low E l values for PLAs and scPLA, compared with other polymers with planar zigzag skeletons in the crystalline regions, were considered to be attributed by helical structure. The difference of the E l values between PLAs and scPLA could be explained by the difference of the helix types, that is, 3 1 helical skeleton of scPLA in the crystalline regions is slightly extended compared with 10 3 helix of PLAs. In addition, these E l values were found to be temperature independent down to cryogenic temperature. The E t values of scPLA were around 4 GPa, which is higher than those (around 3 GPa) of PLLA and PDLA. The higher melting point (∼230 °C) compared with those (∼180 °C) of PLLA and PDLA can be attributed to higher E t value of scPLA, in other words, higher intermolecular interaction of scPLA compared with those of PLLA and PDLA. However, compared with the E t value (4 GPa) of polyethylene where only van der Waals interaction acts, there exist no strong intermolecular interaction for PLAs and scPLA. [ABSTRACT FROM AUTHOR]