The heat of fusion measured with differential scanning calorimetry (DSC) is typically divided by a constant value of the heat of fusion of 100% polyethylene (PE) crystal (DeltaH(m)(o)) for the estimation of the fraction crystallinity of PE copolymers, regardless of the density [i.e., the short-chain branching (SCB) concentration]. In this work, values of DeltaH(m)(o) of about 288 J/g were determined with a combined DSC and X-ray diffraction (XRD) method for a series of PE copolymers containing SCB from 0 to 50 Br/1000 C (density = 0.965-0.865 g/cc). There was no systematic change in DeltaH(m)(o) observed across this density range. This result supports the suitability of determining the fraction crystallinity of PE of any density by the simple division of the observed heat of fusion determined by DSC by a constant value of DeltaH(m)(o). This DSC method yielded values of PE crystallinity in good agreement with corresponding values determined by XRD for a series of PE copolymers. The determination of DeltaH(m)(o) involved a small precision error for higher density (lower SCB) PEs, but the precision error increased for lower density (i.e., higher SCB) PEs. This was due to the difficulty in measuring the heat of fusion for lower density PEs, which exhibited low values of the heat of fusion and melted only slightly above room temperature, and due to the difficulty of measuring lower values of crystallinity by XRD. The crystal thickness measured by small-angle X-ray scattering for this series of PE copolymers decreased exponentially from about 280 to 6 Angstrom.