en Research Paper Research Paper Abstract: Composite specimens were prepared by dispersion of various amounts of nano-sized Al2O3 fillers in a monomer system containing 60% Bis-GMA and 40% TEGDMA. For comparative purposes, composite samples containing micrometer size Al2O3 fillers were also prepared following the same procedure. The mechanical properties of the light- cured samples were assessed by three-point flexural strength, diametral tensile strength, and microhardness tests. The results indicated a more than hundred percent increase in the flexural strength and nearly an eighty percent increase in the diametral tensile strength values in the samples containing nano-size Al2O3 filler particles. It is interesting to note that, this improvement was observed at a much lower nano-size filler content. Fracture surfaces analyzed by scanning electron microscopy, indicated a brittle type of fracture in both sets of specimens. Keywords: Composites, Polymers, Ceramics, Mechanical Properties 25 33 http://ijmse.iust.ac.ir/browse.php?a_code=A-10-3-146&slc_lang=en&sid=1 F. Foroutan 180031947532846001271 180031947532846001271 No J. Javadpou * javadpourj@iust.ac.ir 180031947532846001272 180031947532846001272 Yes A. khavandi 180031947532846001273 180031947532846001273 No M. Atai 180031947532846001274 180031947532846001274 No H. R. Rezaie 180031947532846001275 180031947532846001275 No