High-density polyethylene (HDPE) beads were successfully surface-crosslinked in a modified plasma reactor. The modified plasma reactor treats large amounts of beads, which are uniformly surface-crosslinked. In this study, effects of the gas pressure, radio-frequency (RF) power, and the treatment time on the degree of surface crosslinking were systematically investigated. Degree of surface crosslinking was measured by solvent extraction method (boiling xylene method, BXM). The gel content of plasma-treated HDPE increases from 0.0 to 1.05% within 10 min at 100 mTorr, 200 W. FTIR and DSC analyses show that the crosslinked layer after plasma treatment is limited only at HDPE surface without changing the bulk thermal property of HDPE. Through the analysis of FTIR, it was confirmed that main peaks corresponding to CH2 bands were decreased and two peaks corresponding to CF2 and CF3 were observed after plasma surface modification.