Tensile deformation behaviors of the crystalline regions of poly(trimethylene terephthalate) (PTT) and of PTT films were investigated at temperatures ranging from room temperature down to 18 K. The elastic moduli of the crystalline regions (crystal modulus) were found to be temperature dependent. Measured by X-ray diffraction, the crystal modulus along the chain axis (E,) was 2.59 GPa at 300 K and 5.39 GPa at 18 K, and the crystal modulus in the direction perpendicular to the chain axis (E,) was.3.7 GPa at 300 K and 7.4 GPa at 18 K. This inversion of the usual order of E, and E, values is a result of the extraordinarily small E, due to the very contracted skeleton of PTT and makes a PTT film macroscopically isotropic even after uniaxial drawing up to a draw ratio of 4. The E-1 and the macroscopic storage modulus coincide each other, and both increased with decreasing temperature. Even at 18 K, a drawn PTT film could be strained up to 8% without loading-unloading hysteresis.