A chemical modifier (acrylic acid) was used to improve the thermal properties of polypropylene/ ethylene propylene diene terpolymer/calcium carbonate (PP/EPDM/CaCO3) composites. Treated and untreated PP/EPDM composites were filled by CaCO3 at 0, 20 and 40% wt. The composites were prepared using Z-blade mixer machine at 180oC and 50 rpm of rotor speed. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) methods were used to analyze the thermal properties of the composites. Thermogravimetric analysis indicated that the total weight loss of PP/EPDM/CaCO3 composites decreased with the increasing filler loading for the treated and untreated composites. Meanwhile, the presence of acrylic acid was found to have increased the thermal stability and crystallinity of PP/EPDM/CaCO3 .