AbstractTemperature dependence of the elastic modulus E ₁ of crystalline regions in the direction parallel to the chain axis has been measured by x-ray diffraction for various kinds of polyethylene (PE) with different microstructures. The E ₁ value is 235 GPa for all kinds of PE at room temperature. However, in some cases, E ₁ began to decrease at a certain temperature, in one case at 50°C and in one at 65°C. The lattice spacing for the (002) plane also decreased with increasing temperature. The thermal expansion coefficient αc changed discontinuously around the temperature range where E ₁ began to decrease. This indicates that axial thermal molecular motion (i.e., incoherent thermal motion) is enhanced at high temperature. These phenomena could well be explained with the kinked-chain model where two gauche conformations are incorporated in the all-trans, fully extended conformation. The kinked chain deforms very easily through the change of internal rotation around the C—C bond, which has a small force constant. So it is considered that the decrement of E ₁ at high temperature is due to the incorporation of a small amount of contracted portions in the chain molecules. On the other hand, in some cases, the E ₁ value was constant at 235 GPa up to 110°C In these cases, αc did not change with increasing temperature. Consequently, whether the molecular motion which brings the decrement of E ₁ occurs or not is the origin of the difference of the temperature dependence of E ₁ of PE. The E ₁ value of PE obtained at -155°C is as high as 254 GPa.