Polyethylene nanocomposites containing silica nanospheres, synthesized by the sol-gel method, were produced via in situ polymerization. The silica nanospheres were added together with the catalytic system [metallocene catalyst and methylaluminoxane (MAO) as cocatalyst] directly to the reactor and used for the polymerization of ethylene. The polymerization activity increased slightly in the presence of 1 wt % silica nanospheres in comparison to the homogeneous polymerization sans filler. The Young's modulus of the nanocomposites increased 19-25% without a significant decrease in the elongation at break with respect to the neat polyethylene. The polymer particle morphology was also significantly improved with the incorporation of silica nanospheres. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1771-1780, 2011