In the present study different series of high-density polyethylene (HDPE) nanocomposites, containing 0.5, 1, 2.5 and 5wt.% of fumed silica (SiO 2 ) 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 SiO 2 content. Tensile strength increases by increasing silica content up to 2.5wt.% SiO 2 , while at 5wt.% a reduction was observed. Additionally, Young's modulus increases continuously while impact strength has the opposite trend and SiO 2 content has no monotonic effect on HDPE melting point. Thermal stability of HDPE was enhanced due to the incorporation of SiO 2 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. SiO 2 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 SiO 2 nanoparticles.