First, some groups of C=O and C–O were introduced onto high-density polyethylene (HDPE) chains by ultraviolet irradiation for short time in ozone atmosphere. The content of the groups was increased with increasing the irradiation time. The irradiated HDPE was blended with calcium carbonate (CaCO₃) to prepare the composites. The melting temperature and crystallinity of HDPE in irradiated HDPE/CaCO₃ composites were lower than the HDPE in HDPE/CaCO₃ composites. Compared with the HDPE/CaCO₃ composites, the dispersion of CaCO₃ and the interfacial interaction between CaCO₃ and HDPE in the irradiated HDPE/CaCO₃ composites increased respectively. With increasing the irradiation time, the mechanical properties (especially impact strength) of the irradiated HDPE/CaCO₃ composites were markedly enhanced, while their thermal stability decreased slightly. For example, the tensile strength and impact strength of the irradiated (20 min) HDPE/CaCO₃ composites increased from 25.7 MPa and 72 J m^–1 to 30.3 MPa and 416 J m^–1, respectively, compared with those of the HDPE/CaCO₃ composites, and stiffened and toughened HDPE composites were obtained.