Epoxy composite materials filled with nano-alumina particles were prepared by mechanical mixing techniques. The glass transition temperatures (T (g)) of the nanocomposites were found to decline significantly with the increasing filler content. After the addition of 30-phr nanoparticles, the T (g) of the filled sample decreased by as high as 55 A degrees C, as compared with that of the neat epoxy polymer. Based on the selective adsorption hypothesis and the molecular diffusion, it is speculated that the hardener molecules were unevenly distributed in the nanocomposites, which caused imbalanced stoichiometry between the epoxy and the hardener and finally decreased the T (g). Some results that may support the adsorption hypothesis were given and discussed.