We have used IR spectroscopy to study the effect of Cu nanoparticles on the degree of crystallinity in nanostructured high-pressure polyethylene (PE) films. We have shown that for ∼1 mass % Cu in the polyethylene films, we observe the maximum degree of crystallinity. This is connected with commensurability between the nanoparticle sizes and the periodicity of the three-dimensional structure of the straightened polyethylene chains in its transitional phase. Dielectric measurements on the nanostructured polyethylene films are support for the features observed in the IR spectra for the systems under study. From the data obtained, we hypothesize that we can control degree of crystallinity of the polymer by varying the sizes of the nanoparticles and the type of polymer.