Morphological structure and other properties of polyethylene photocross-linked in the melt (XLPE) have been studied by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), density, and swelling measurements. The WAXD measurements show that the crystallinity of polyethylene decreases with increasing gel content in the XLPE samples and that the regularity and the size of crystals are affected by photocross-linking. DSC data confirm that photocross-linking occurs preferentially in the amorphous and interfacial regions and causes depression of the melting point and decreases the heat of fusion. The density of XLPE apparently decreases with increasing gel content, which is due to decreased crystallinity and lattice expansion. The cross-link density determined by the swelling measurements and evaluated using the Flory–Rehner equation correlates well with the changes in crystallinity and melting point. © 1993 John Wiley & Sons, Inc.