High density polyethylene (HDPE)/metallocene linear-low density polyethylene (m-LLDPE) blends and high density polyethylene/conventional (Ziegler-Natta) linear low-density polyethylene (c-LLDPE) blends were prepared at various compositions. The first part of this study involved these blends by melt mixing in a single screw extruder with the composition of 0-100% by weight of m-LLDPE or c-LLDPE and HDPE. Thermal properties of each blend had been investigated to elucidate miscibility by using a differential scanning calorimeter (DSC)and a dynamic mechanical thermal analyzer (DMTA). The blend composed of 60-100% by weight of m-LLDPE or c-LLDPE was separately blown to the heavy duty films with 150, 165 and 180 micron thickness. All these films were subjected not only to the mechanical testing, i.e., tensile strength, elongation at break, impact resistance, and seal strength but also to optical testing for film clarity. These results indicated that HDPE/c-LLDPE and HDPE/m-LLDPE blends were miscible in both crystalline and amorphous phases. The mechanical properties of all HDPE/m-LLDPE films were superior to HDPE/c-LLDPE films at the same composition and c-LLDPE films. The downgauging of HDPE/m-LLDPE film was applicable due to the better mechanical properties of HDPE/m-LLDPE films at 90% of m-LLDPE with 150 or 165 micron thickness than c-LLDPE films at 180 micron thickness. The second part of this study was to match the mechanical property of HDPE/LLDPE blend with medium-density polyethylene (MDPE). The compositions of 0-100% by weight of m-LLDPE or c-LLDPE were prepared by melt mixing in single screw extruder. Several blends of HDPE/m-LLDPE and HDPE/c-LLDPE blends at the composition which showed the same melt flow index (MFI) and density with MDPE were selected for blowing as the films with 40, 60 and 80 micron thickness. All these films were subjected to the mechanical testing. The results revealed that the equivalent mechanical properties of HDPE/m-LLDPE films and HDPE/c-LLDPE films to MDPE films of the same thickness can be achieved.