Silica nanoparticles were prepared by ultrasound-assisted and conventional sol-gel method. The synthesis procedures were designed and optimized by the Taguchi experimental design method. Molar concentrations of TEOS, H(2)O, NH(4)OH, and reaction temperature were chosen as main factors. The results showed that the molar concentration of ammonia is the main factor which affects the particle size of the silica nanoparticles. The chemical structure, size, and morphology of the product were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, laser light scattering, and scanning electron microscopy. By the optimum conditions of the ultrasound-assisted sol-gel method, silica nanoparticles with an average particle size of 13 nm were prepared.