A comparative study was undertaken on well characterized HDPE and LDPE samples in order to obtain a better understanding of the morphology of the crystalline phase. Thickness of the lamellae was determined by WAXS. The melting points were calculated by empirical and Gibbs-Thomson equations, considering either folded or extended chains, and compared to DSC data. The results confirm that in HDPE the chains are folded, the segment between folds containing about 100 carbon atoms, while in LDPE, the chains are extended, with segments of about 73 carbon atoms in the crystalline core, linked to the paracrystalline layer and amorphous zones, which contain long folds and chain ends, as proposed in the 3-phase model by Vile et al.