AbstractWe prepared cycloolefin copolymer (COC)/fumed silica nanocomposites by melt compounding to study the effect of the filler dimensions (filler surface area) on the physical properties, with particular attention to their thermal, mechanical, and optical behaviors. Thermogravimetric analysis revealed a positive contribution of silica nanoparticles to the thermal degradation resistance of COC, as the decomposition temperature of the nanofilled samples increased by 40°C with respect to that of the unfilled matrix. Dynamic mechanical thermal analysis and quasi‐static tensile tests of the nanocomposites evidenced a slight stiffening effect, proportional to the nanofiller surface area, without any reduction in the fracture toughness. Creep resistance of the nanocomposites was increased by the addition of silica nanoparticles, especially when high‐surface‐area nanoparticles were used. The positive effect of the nanoparticles on the viscoelastic and fracture behavior was related to the uniform dispersion of silica aggregates in the matrix. Ultraviolet–visible spectrometry measurements evidenced that the original transparency of neat COC was practically maintained after the addition of silica nanoparticles. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011