The role of silica fumed nanoparticles, having different surface area and surface treatments, on the fracture behaviour of a linear low density polyethylene (LLDPE) matrix was investigated. Tensile tests under quasi-static and impact conditions evidenced beneficial effects on both the elastic modulus and the strain at yield. By the essential work of fracture approach, increments of both the initiation and the propagation components of the specific essential work of fracture terms were highlighted. Dilatometric measurements excluded the filler-matrix debonding as a possible toughening mechanism, while a progressive alignment of silica aggregates was detected by TEM observations.