Two types of nanosilica (NS) particles with different average particle sizes (20 nm and 80 rum in diameter, respectively) were used to fabricate epoxy-silica nanocomposites (ESNs) in this study. No significant differences in fracture behavior were observed between the epoxies filled with 20 nm NS particles and the epoxies filled with 80 nm NS particles. Interestingly, both types of NS particles were found to be more efficient in toughening epoxies than micron size glass spheres. As with micron size glass spheres, the fracture toughness of the ESNs were affected by the crosslink density of the epoxy matrix, i.e. a lower crosslinked matrix resulted in a tougher ESN. The increases in toughness in both types of ESNs were attributed to a zone shielding mechanism involving matrix plastic deformation. Moreover, the use of Irwin's formalized plastic zone model precisely described the relationship between the fracture toughness, yield strength and the corresponding plastic zone size of the various ESNs examined. (C) 2009 Published by Elsevier Ltd.