In the present study different series of high-density polyethylene (HDPE) nanocomposites, containing 0.5, 1, 2.5 and 5 wt.% of fumed silica (SiO2) nanoparticles were prepared by melt-mixing on a Haake-Buchler Reomixer. From SEM micrographs it was found that even though there is a fine dispersion of nanoparticles into HDPE matrix there are also some aggregates formed and their size depends directly on the SiO2 content. Tensile strength increases by increasing silica content up to 2.5 wt.% SiO2, while at 5 wt.% a reduction was observed. Additionally, Young's modulus increases continuously while impact strength has the opposite trend and SiO2 content has no monotonic effect on HDPE melting point. Thermal stability of HDPE was enhanced due to the incorporation of SiO2 nanoparticles. From the kinetic analysis of thermal decomposition of HDPE and its nanocomposites, it was concluded that in order to describe the thermal degradation mechanism, two consecutive mechanisms of nth-order (Fn) and nth-order with autocatalysis (Cn) have to be considered. SiO2 have no effect on decomposition mechanism but only to the activation energies, which in nanocomposites are higher, compared with neat HDPE, due to the stabilization effect of SiO2 nanoparticles. (C) 2008 Elsevier B.V. All rights reserved.