Characterization of pore structure and pore wall crystal structure was performed on porous high-density polyethylene (HDPE) material using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction (ED). The porous HDPE material was obtained through crystallization from swollen cross-linked polyethylene gels (CSX process(1)) in supercritical propane. SEM showed an open-pore structure of micron-sized pores, large void fraction, and surface area as well as thin yet rigid pore walls, making this material a good candidate for a variety of applications. TEM revealed oriented lamellar structure in the pore walls which was much different from structures found in typical bulk HDPE as well as that of the cross-linked HDPE before CSX processing. Electron diffraction results confirmed the presence of oriented lamellar stacking. On the basis of this oriented lamellar structure, possible mechanisms for crystallization and pore formation are suggested.