We have studied the effect of filler particle size and packing density on mechanical properties of experimental dental composites. Commercially available monodispersed silica spheres of 200, 250, 500 and 1500 nm in diameters were dispersed by manual mixing, followed by sonication into a photocurable 50/50 BisGMA-TEGDMA resin. The mixture was cast into 6 x 50 mm cylindrical glass molds, photocured for 6 min and postcured for 24 hrs at 37�C. Packed specimens were prepared by settling in a centrifuge at 5000 rpm for 10 min before photocuring. Silica content was fixed at 60wt%. It was found that, for unpacked specimens, the particle size had little effect on modulus though yield and compressive strengths increased with decreasing particle size. Packed composites had a modulus of around 7.5 GPa compared to 6.5 GPa for those without packing. This is statistically significant, indicating that packing under centripetal force does enhance modulus. When 20wt% 13nm nanosilica from Highlink-OGTM (Clariant) was packed in combination with 40 wt% monodisperse beads, the modulus increased to 8 GPa. These results demonstrated that combining nanosilica with monodisperse silica spheres under centrifugation significantly enhances mechanical properties of the nanocomposites.