Ultra-high-molecular-weight polyethylene (UHMWPE) fibers were treated by photochemical bromination. The analysis of the fibers by XPS and ATR-FTIR showed that this process led to the introduction of C-Br and C-OH moieties and generated C=C bonds at the PE fiber surface. Composites were fabricated using either treated or untreated fibers and high-density polyethylene (HDPE) for the matrix. WARD analysis showed that the treated fibers, through offering a higher concentration of crystallization nuclei, generated a denser transcrystalline layer with higher specific radial orientation with respect to the fiber axis-compared with the untreated fiber. Furthermore, the introduction of polarity onto the fiber surface enabled analysis of the complex relaxation behavior of PE/PE composites by dielectric spectroscopy. It showed the typical alpha, beta and gamma-relaxation processes of polyethylene, combined with the effect of the transcrystalline layer, generating-among other changes-a strong beta-transition.