AbstractEpoxy/Al2O3 nanocomposites were prepared using an epoxy resin, diglycidyl ether of bisphenol A, and cured with a polyoxypropylene diamine (Jeffamine D‐400). Transmission electron microscopy and wide angle X‐ray diffraction were employed to reveal the morphology of epoxy/Al2O3 nanocomposites. Dynamic mechanical analysis results showed that the storage modulus and the glass transition temperature (Tg) of epoxy were improved. Tensile strength and Young's modulus also increased with increasing Al2O3 loading. Fracture toughness, as indicated by the stress intensity factor, KQ, was determined using single edge notch bending method, and 40% increase in KQ was observed with only 2 vol % Al2O3. Scanning electron microscopy study of fracture surface showed a rather smooth and flat morphology for neat epoxy. However, massive plastic deformation was observed for epoxy/Al2O3 nanocomposites, leading to the significant increase in fracture toughness. The influence of spherical Al2O3 nanoparticles on thermophysical properties of epoxy was discussed and compared with that of sheet‐like nanoclays. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1466–1473, 2006