The structure and hydrophilicity of high-density polyethylene (HDPE) functionalized by ultraviolet irradiation in air and O-2, were studied with Fourier transform infrared, X-ray photoelectron spectroscopy, differential scanning calorimetry, and contact-angle measurements with water and liquid paraffin. The results showed that C-O, C(=O)O, and C=O functional groups were introduced onto molecular chains of HDPE during irradiation in air and O-2, and their contents increased with the irradiation time. The number of oxygen-containing groups of HDPE irradiated in O-2 was much higher than that in air. A higher functionalization efficiency for HDPE was achieved by irradiation in O-2 After the irradiation, the contact angle of HDPE with water became smaller, the contact angle of HDPE with liquid paraffin became larger, and the surface tension increased; this showed that the hydrophilicity of irradiated HDPE was improved. HDPE irradiated in O-2, exhibited better hydrophilicity than HDPE irradiated in air. Furthermore, with an increase in the irradiation time, the melting temperature of HDPE decreased, and the heat of fusion and the degree of crystallinity increased. (C) 2003 Wiley Periodicals, Inc.