The elastic modulusE1of crystalline regions of isotactic poly(4-methyl-1-pentene) (P4M1P) in the direction parallel to the chain axis and its temperature dependence were investigated by x-ray diffraction.E1values for the (001) and (007) planes of P4M1P agreed well at 4.1 GPa at room temperature. This smallEvalue could be attributed both to the contracted helical conformation (7/2 helix) and to a large cross-sectional area of P4M1P in the crystalline regions. Though the lattice distortion of the first kind increased and partial destruction of crystallites occurred with increasing stress, the stress-strain curve for the crystal lattice was linear up to 80 MPa. However, theE1value remained unchanged up to 150°C. The lattice spacing for the (007) plane increased monotonously with increasing temperature, with a thermal expansion coefficient of 0.9 × 10−4K−1. These phenomena were in contrast with the temperature dependence ofE1values of various polymers previously reported, in whichE1decreased in the temperature region where the thermal expansion coefficient changed. It is considered for P4M1P that the axail thermal molecular mitions which cause the decrement of E1do not take place and that the deformation mechanism of the chain molecules remains unchnaged in the crystalline regions over the temperature and stress range studied.