The strain-optical coefficient and relaxation modulus were simultaneously measured of high density polyethylene at various temperatures ranging from 12 to 100°C. Not only the horizontal but also the vertical shifts Were necessary to obtain smooth master curves by application of time-tem perature superposition. However, the relaxation modulus decreases with rising temperature, While the strain-optical coefficient increases, indicating that the variation of the relaxation modulus and the strain-optical coefficient with time can not be explained by decrease in crystallinity With rising temperature, since decrease in crystallinity usually causes decrease in the strain-optical coefficient. It is required that some other explanation will be sought for the vertical shift in the time-temperature superposition of the time-dependence curves of the relaxation modulus and the strain-optical coefficient at various temperatures. The master curve of the strain-optical coefficient or the optical distribution function of relaxation times determined from it serves to distinguish the type and thermal history of the polyethylene.