HDPE/CaCO3/LDPE-g-MA composites of high density polyethylene (HDPE) and calcium carbonate (CaCO3) with maleic anhydride grafted low density polyethylene (LDPE-g-MA) as a compatibilizer were prepared by melt mixing. LDPE-g-MA was prepared using a solution process. The maximum grafting degree was obtained at 6 wt% maleic anhydride (MA) and 0.2 wt% dicumyl peroxide (DCP) at 120 ℃ and 240 min. Functional groups not found in pure LDPE were observed with the formation of LDPE-g-MA. The successful dispersion of CaCO3 particles in the HDPE matrix using LDPE-g-MA was revealed. The crystallite size of the composites was a little higher than that of CaCO3. The mechanical properties of the HDPE/CaCO3/LDPE-g-MA composites increased with decreasing CaCO3 content. Thermogravimetric analysis (TGA) revealed a higher thermal stability for the HDPE/CaCO3/LDPE-g-MA composites than for pure HDPE. Differential scanning calorimetry (DSC) revealed that the crystallization conditions were not substantially different. However, melting enthalpy and crystallinity increased with decreasing CaCO3 content. The thermal stability was greatly improved compared to that previously reported for nano CaCO3.