The molecular-topological structure of powdered high-density polyethylene (PE) after its compression under an optimized pressure and γ -radiolysis was first analyzed. A semicrystalline structure composed of one amorphous block and three crystalline modifications was formed with different rates and different melting onset temperatures in unirradiated PE and in PE irradiated to a dose of 5.5 Mrad. This structure was retained to 242 Mrad; after such a dose, the amorphous block completely disappeared. The net crystallinity of PE at a dose of 5.5 Mrad reached a minimum value but was close to 100 wt % in the initial state or after irradiation to 242 Mrad.