The present study is aimed at investigating differences in mol. structure, crystallinity, and morphol. between uncompatibilized and compatibilized blends of high-d. polyethylene (HDPE) and nylon 12 by using Fourier-transform (FT) Raman spectroscopy, wide-angle X-ray diffraction, differential scanning calorimetry, and SEM. Uncompatibilized and compatibilized blends of HDPE/nylon 12 with a nylon 12 content ranging from 10 to 90 wt.-% with increments of 10 wt.-% were prepd. The compatibilized polymer blends were prepd. by adding a small amt. of maleic anhydride (MAH) and it was found that 0.5 wt.-% MAH yielded a good dispersion. SEM images show that both kind of blends have a different miscibility behavior. The uncompatibilized and compatibilized blends yield quite different X-ray diffraction patterns; the latter blends with nylon 12 content > 70 wt.-% show orientational effects in the X-ray pattern of the HDPE. The crystallinity of the HDPE of both blends was evaluated by the full width at half intensity of the (110) reflection of HDPE. To do that, the diffraction peaks were analyzed by a curve-fitting method. To evaluate the crystallinity from Raman spectra, the intensity ratio of the two bands at 1129 and 1110 cm-1 was used. Of note is that the 1129 cm-1 band is caused by a sym. C-C stretching mode of all-trans -(CH2)n-groups arising only from HDPE. The Raman spectra and X-ray diffraction measurements revealed that when the nylon 12 content reaches 70 wt.-%, the crystallinity of HDPE in the compatibilized blends becomes higher than that of HDPE in the uncompatibilized blends. This result is different from the general trend of crystallinity of HDPE in polymer blends. The difference suggests that the effect of the high viscosity of the nylon-rich phase on the crystallinity is more significant than the effect of the impurity (MAH-grafted PE). It seems that the extension of the nylon 12-rich phase during the extrusion process leads to orientational effects because of the increase in the interaction between MAH and HDPE.