High density polyethylene was melt compounded with various untreated (hydrophilic) or surface treated (hydrophobic) fumed silica nanoparticles, having different surface areas. The thermo-mechanical properties of the resulting nanocomposites have been thoroughly investigated. Field emission scanning electron microscopy revealed that nanofiller aggregation was more pronounced as the silica surface area increased, while nanofiller dispersion improved with a proper filler functionalization. The homogeneous distribution of fumed silica aggregates at low filler content allowed us to reach remarkable improvements of thermal stability, evidenced by an increase of the degradation temperature and a decrease of the mass loss rate with respect to neat matrix, especially when surface treated nanoparticles were utilized. Interestingly, the stabilizing effect produced by fumed silica nanoparticles was accompanied by noticeable enhancements of the ultimate tensile mechanical properties, both under quasi-static and impact conditions. Concurrently, a progressive enhancement of both elastic modulus and tensile stress at yield with the filler amount, was observed.