Elastic modulusE1of crystalline regions in the direction parallel to the chain axis has been measured for various kinds of polyethylene (PE) with different microstructures, such as ultrahigh-modulus PE, linear low-density PE, high-pressure-crystallized PE, and a heat-drawn PE whose crystallite size exceeds its long period, etc., by x-ray diffraction. TheE1value, which corresponds to the initial slope of stress-strain curve for the crystal lattice, is 235 GPa for all kinds of PE irrespective of the estimation method of the diffraction peak shift for the (002) plane owing to the constant stress applied. This indicates that the assumption of “homogeneous stress distribution,” that is, stress in the crystalline regions was equal to the stress applied to the sample, is proved experimentally at least for PE. TheE1value at −155°C is obtained as high as 254 GPa. The x-ray diffraction method is considered to be the most proper method to evaluate theE1value.