Uniaxially oriented films of linear medium-density polyethylene and samples subjected to high pressure during heating were investigated by methods of x-ray structure analysis, scanning electron and optical microscopy, dynamic mechanical testing, differential scanning calorimetry, and density measurements. It was found that the polymers can be readily oriented up to high drawing ratios and that they possess improved mechanical characteristics. The structure and morphology of such material appeared to be typical of superhigh-modulus polymers. Heating of the original sample under pressure above 4–5 kbar results in a phase transition of the orthorhombic crystal into the pseudohexagonal mesophase. The crystallinity of the cooled material amounts to almost 100%; the longitudinal dimensions of the crystallites are up to 100 run. The pressure-crystallized material is more perfect than the highly drawn material.