Addition of nanoparticles to polymers like PE, leads to improvement of physical, mechanical and thermal propertieses of them. On the other hand, usually addition of nanoparticles causes the reduction of impact properties of polymers. To avoid the reduction of impact strength, it is necessary to use an impact modifier simultaneously with addition of nanoparticles. In this research, nanocomposites of HDPE/Nanosilica containing two kinds (Hydrophobic and Hydrophilic) and different amount of nanosilica were produced via melt mixing, using an internal mixer. Ethylene-Octene Copolymer (EOC) was used as an impact modifier for this system too. Mechanical properties of nanocomposites were studied by tensile and impact tests. The results showed that all HDPE/Nanosilica containing un-functionalized (Hydrophilic) nanosilica have lower mechanical properties than neat HDPE. On the other hand, all nanocomposites containing functionalized (Hydrophobic) nanosilica, have higher elastic modulus which are increased by increasing the amount of nano particles. The tensile strength of these samples showed the same trend as well, but the elongation at break and the impact strength of these nanocomposites were decreased. Finally nanocomposite samples containing a small amount of EOC, showed great improvement of impact strength, but the reduction of stiffness. It was found that the optimum amount of EOC for this system is lower than 5 wt. % which leads to improvement of both tensile and impact properties. Keywords: HDPE, EOC, Nanosilica, Mechanical Propertieses