This study determined the effect of bleaching agents on the fracture toughness of composite materials. Four nanofilled resin composites were evaluated: Filtek Supreme Plus, Tetric EvoCeram, Premise and Esthet-X. Four concentrations of bleaching agents were tested: Opalescence PF 10%, 20%, 35% and 45%. Fifty specimens of each composite conforming to ASTM guidelines for the single edge notch bar-shaped specimen were fabricated in a metal mold. The specimens were stored in artificial saliva at 37 degrees C for a minimum of 24 hours prior to exposing them to the bleaching agents. Ten specimens of each material were used as controls. Forty specimens of each brand were evenly divided into four groups for bleaching agent application (n = 10). The resin composites were bleached on both sides for 14 days. The specimens were then subjected to a three-point bending test with a crosshead speed of 0.2 mm per second. The fracture toughness (K(Ic)) was calculated. The control group K(Ic) value of Esthet-X was significantly higher than that of the other composites in the controlled groups. Bleaching agents significantly improved the fracture toughness values of Filtek Supreme Plus. The application of bleaching agents did not significantly change the fracture toughness values of the other nanofilled resin composites tested.