Nano-sized particle filled polymer composites have been received great attention of researchers and industrial institutions in recent years due to their unique properties, save as high mechanical strength, thermal and solvent resistance as compared to traditional composite materials. In this study, calcium carbonate (CaCO3) filled polypropylene (PP) and Polyethylene (PE) composite blends were prepared using a co-rotational twin screw extruder with a calcite particle content varying from 0 to 30 wt. % . Tensile and three-point bending test coupons were prepared by injection moulding using the extruded composite blends. The effects of calcite reinforcement (with and without stearic acid treatment) on the microstructural, thermal and mechanical properties of neat PP and PE were investigated. Nano-CaCO3 powders were characterized by means of Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The PE and PP were characterized via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Analytical results were compared with the experimental results.