Abstract: Light-cured composite resins were prepared by illuminating a cylinder with a diameter of 4 mm and a length of 6 mm for 40 s, using bisphenol A ethoxylate dimethacrylate as monomer, triethyleneglycol dimethacrylate as diluting agent, camphoroqui-none and dimethyaminoethyl methacrylate as photoinitiators and nanodiamonds as reinforcements. The nanodiamonds were of different size and were surface-modified by methacryloxypropyltrimethoxy silane. The flexural strength, elastic modulus and other mechanical properties of the resins were investigated to determine how the nanodiamonds influenced the properties of the composite res ins. Results showed that the nanodiamonds significantly improved flexural strength and elastic modulus of the composite resins, but also decreased their depth of lightcuring. A percolation phenomenon was found in the composite resins, which indicated that their mechanical properties decreased significantly when the nanodiamond content was greater than a value depend