In the current research, physical and morphological properties of high-density polyethylene (HDPE) were investigated after equal-channel angular pressing (ECAP) for up to three passes via route A, in which the sample is pressed without any rotation between individual passes, at the temperature of 80 degrees C. The results of density, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) represent a decrease in the polymer's crystallinity after deformation, which is more evident after three passes of ECAP. Moreover, a phenomenological mechanism is proposed to explain the variations of crystallinity after ECAP processing. The occurrence of an additional crystalline peak in the XRD pattern, as well as melting doublet in the DSC curve of three-pass ECAP-deformed sample, demonstrate that some part of the initial orthorhombic crystalline phase of HDPE has been transformed to monoclinic crystal structure after ECAP deformation. Furthermore, a reduction in melting temperature and broadening of crystalline peaks show that the thickness of crystalline lamella may decline in one-pass ECAP-deformed sample, which could significantly affect dynamic mechanical behavior of HDPE. In addition, dilatometry results and observation of the impact-fractured surfaces of deformed samples reveal that oriented structures formed in amorphous and crystalline regions of the workpiece.