The degree of crystallinity and unit cell parameters have been determined using WAXS on a number of compression molded high-density polyethylene (HDPE) plaques processed at widely varying conditions of crystallization and annealing times and temperatures. Changes in unit cell parameters with variations in processing conditions can be explained in terms of increases in lamellar thickness of polyethylene crystals with increasing thermal treatments. Concomitant increases in the degree of crystallinity of these samples can also be explained in terms of lamellar thickening and other changes in polyethylene morphology. Crystallinity determinations using XRD data are also compared with values determined by differential scanning calorimetry (DSC) and solid-state 13C nuclear magnetic resonance (NMR). Comparisons of crystallinity values obtained by these three different techniques can reveal details of the morphology of HDPE including the presence of an interfacial zone in addition to the crystalline and amorphous components of the system.