Crystal structure of uniaxially oriented polyethylene has been analysed as a function of temperature by using an X-ray imaging plate system equipped with Software useful for the structure analysis of polymer crystals. The cell parameters a and b, the anisotropic temperature factors of the carbon atom, and the setting angle of the planar-zigzag chain measured from the b axis, etc., were found to show a deflection point around +10 degrees C. The structure and thermal motion below this characteristic point could be interpreted in terms of harmonic vibration. But, above this deflection point, the thermal motion became more remarkable, requiring an introduction of an idea of anharmonic vibration. The temperature dependence of these structural parameters is different between the samples used in the measurement. The sample oriented at relatively low draw ratio showed larger temperature dependence and a clearer deflection point around 10 degrees C, compared with the case of the ultradrawn sample. This difference may reflect a difference in the remnant strain in the samples.