The crystallinity of non-molten and pre-molten uncompatibilized and compatibilized polymer blends of high d. polyethylene (HDPE)/Nylon 12 have been investigated by using FT-Raman spectroscopy, differential scanning calorimetry (DSC), and wide angle X-ray diffraction (WAXD). The FT-Raman, DSC, and WAXD measurements have revealed that the crystallinity of HDPE of both uncompatibilized and compatibilized blends increases upon melting except for the compatibilized blend with the Nylon 12 content of 80 wt%. The degree of the increase is significantly larger for the uncompatibilized blends than the compatibilized blends. The FT-Raman data suggests that the crystallinity of the compatibilized blend with the Nylon 12 content of 80 wt% decreases slightly after melting. It is very likely that the compatibilized polymer blends are well oriented during the melting and molding process by an extruder because of the existence of maleic anhydride (MAH)-grafted copolymer as a compatibilizer. In contrast, it seems that the uncompatibilized polymer blends are not so much oriented and have more amorphous phase in the Nylon 12 rich region before melting. Thus, the melting process induces a recrystn. process.