Calcium carbonate (CaCO3) with various shapes and size was prepared using CaCl2 and Na2CO3 aqueous solutions containing various additives. High density polyethylene (HDPE)/4 wt% CaCO3 composites were prepared using a Haake Rotational Rheometer. The effects of the CaCO3 particle shapes and size on the thermal and mechanical properties were investigated and characterized by differential scanning calorimetry (DSC) and universal testing machine. The results show the addition of CaCO3 to HDPE can increase the crystallization and decrease the melting temperature and crystallization temperature. The tensile yield strength of the composites was higher than that of HDPE. Sphere-like CaCO3 is the best form in improving the thermal stability and cubic CaCO3 can cause negative effect to the toughness. The impact strength of the composites is significantly increased with decreasing CaCO3 particle size. SEM examination of the fractured surfaces of the composites shows that CaCO3 reacted in cavitations and "lance" functions which increase the impact strength of the HDPE/CaCO3 composites.