High density polyethylene (HDPE) beads were surface-crosslinked in a modified plasma reactor by using argon plasma. The modified plasma reactor can treat a large amount of beads to be uniformly surface-crosslinked. The objectives of this study were to develop a shape-stabilized functional thermal energy storage material and to find an optimum plasma reaction condition out of various operations. To achieve these objectives, we systematically studied the effects of the gas pressure, the radio frequency (RF) power and the treatment time on the degree of crosslinking. The degree of crosslinking was measured by solvent extraction method (BXM: boiling xylene method). The chemical and physical characterization of the material was performed by using fourier transform infra-red (FT-IR) spectroscopy, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). Finally, we confirmed the thermal stability of surface-crosslinked HDPE through about 50 thermal cycling tests.