The mechanical properties of CaCO3 filled high density polyethylene (HDPE) compatibilized with ultraviolet (UV) irradiated HDPE (uHDPE) were studied and the interfacial interactions between CaCO3 and the polymers were evaluated by SEM, the Molau test, and ESCA. UV irradiation in air introduces the following functional. groups on HDPE: >COOH, >C=O, -OOH and -OH. The concentration of these groups increases with irradiation time. The addition of a small amount (<10% by wt) of uHDPE to the HDPE/CaCO3 improves the tensile and impact strength. For example, the addition of 3% HDPE irradiated for 500 hours (u500HDPE) increases the tensile and impact strength of HDPE/CaCO3 from 16.7 MPa and 210 J/m to 25.2 MPa and 430 J/m, respectively. Chemical reactions between uHDPE and CaCO3 promote interactions between HDPE and CaCO3. The morphology of the compatibilized compound is nearly homogenous, and no CaCO3 Sediment was observed in a hot xylene solution of HDPE/u500HDPE/CaCO3 when the content of u500HDPE exceeded 30% (wt).