The isothermal crystallization process of n-hexatriacontane (C36H74) and high-density polyethylene (HDPE) was observed by simultaneous DSC/FT-IR and DSC/WAXD measurements. For C36H74, the formation of the trans-conformation and the crystallization exothermic peak of the hexagonal phase from the melt were observed at the same time. The absorption band due to the rocking mode vibration of the methylene group was split at the transformation from the hexagonal phase to the modified monoclinic phase, In the case of HDPE, the formation of the trans-conformation started before the observation of the crystallization exothermic peak. When the crystallization exothermic peak was observed, about 60% of trans-conformation was already formed, The conformational transition from the gauche-form to the trans-form was recognized as a phenomenon occurring before the crystal growth of HDPE.