In the of present work, 2, 5 and 7 wt.% of the Nanosilica were added to High Density Poly Ethylene (HDPE) and Ethylene,1- Octene Copolymer (EOC) respectively. Nanocomposites of HDPE/EOC blends with different compositions, containing 5 wt.% of Nanosilica were prepared too. All nanocomposites were prepared via melt mixing using an internal mixer at 180 and 60 rpm. The rheology, crystallinity and mechanical properties of all samples were studied. It was found that the addition of 5 wt.% of Nanosilica, to HDPE, leads to formation a network in it, which was confirmed by a viscosity upturn and a plateau in storage modulus against frequency diagrams. In other hand, even by addition 7 wt. % of nanoparticles to EOC matrix, there was no evidence of a network formation. DSC results showed that the addition of Nanosilica does not effect on crystallization temperature in both HDPE and EOC matrices, which is due to low surface area of this nano particle. In other hand, the amount of crystallinity is much lower in HDPE nanocomposite in comparison with neat polyethylene, which is due to the formation of network as confirmed by rheology too. The rheological data on HDPE/EOC blend nanocomposites, also DSC results of these samples showed that nearly all nanoparticles are placed in EOC phase. Tensile tests confirmed that the tensile strength and modulus of HDPE and EOC are increased by increasing the amount of nano particles and the elongation at break is decreased. It was also found that the blends of HDPE/EOC have lower mechanical properties in comparison with neat PE, but the addition of nanoparticles improve the strength of these blends. Keawords: HDPE, EOC, Nanosilica, Nanocomposite, rheology, crystallinity