The research is aimed to investigate the mechanical behaviors of epoxy based nanocomposites reinforced with spherical nanoparticles. Five different contents of silica nanoparticles, 5 10 15 20 and 40 wt %, were introduced in the samples. Through a sol-gel technique, the silica particles with diameter of 25 nm were dispersed uniformly into the epoxy matrix. Two different epoxy matrix systems, one is ductile and the other is brittle, were considered to form the nanocomposites and the effect of nanoparticles on the tensile, flexural and fracture behaviors of the two nanocomposite systems were investigated. The experimental results obtained from tensile tests indicate that, in both matrix systems, the modulus of nanocomposites increases with the increment of particulate inclusions and the enhancing behavior is coincided with the model predictions obtained from the Mori-Tanaka micromechanical model. In addition, the fracture tests conducted on single-edge-notch bending specimens reveal that the inclusion of nanoparticles can effectively increase the fracture toughness of the nanocomposites. Furthermore, the extent of the enhancement is more appreciable in the brittle matrix system rather than in the ductile matrix system. The enhancement is attributed to the formation of plastic deformation together with the nanoparticle debonding which dissipate more fracture energy.