The fracture toughness of dental composites containing zirconia nanoparticles dispersed in a bisphenol A glycol dimethacrylate-based monomer blend (GTE) was studied for several yttria contents. Three-point bend test bars with and without a notch were tested at ambient temperature to determine elastic modulus, flexure strength, and fracture toughness. The ZrO2 nanoparticles increased the fracture toughness of the nanocomposites compared to previous results for the matrix and Schott glass-filled nanocomposites. X-ray diffraction analyses revealed mostly tetragonal ZrO2 in the nanocomposites before and after testing, in agreement with a theoretical analysis. The enhancement in fracture toughness in ZrO2-filled nanocomposites was caused mainly by the higher values of particle toughness and interface toughness in GTE/ZrO2 compared to those of GTE/Schott glass nanocomposites.