We investigated in-situ conformational reorganization in a quenched high-density polyethylene solid during heat treatments at various temperatures using in-situ Raman spectroscopic technique. The heat-treatment cycle included three steps: heating from room temperature, maintaining at a fixed sub-melting temperature (tempering), and cooling to room temperature. The heating process caused a conformational transformation from orthorhombic crystalline trans conformers to non-crystalline long trans segments outside the crystalline lattice, which were then amorphized. During the tempering or isothermal duration region, the amorphized long trans segments were steadily subjected to crystalline rearrangement. Finally, the cooling process modified the long trans component (including the amorphized component) into crystalline trans conformers. Overall, a single heat-treatment cycle increased the crystallinity through reorganization of non-crystalline long-trans segments without changing the interfacial content where the original amorphous content remained the same.