Silica (SiO2) nanoparticles were synthesized by the oxidation of tetraethyl-orthosilicate (TEOS) in the bench-scale diffusion flame rector. The modified burner composed of five concentric tubes was developed for the diffusion flame, and used to produce several tens of grams of silica nanoparticles per hour. Flame characteristics of the diffusion flame at the modified burner outlet were investigated. TEOS concentration, total gas flow rate, and gas composition were chosen as key experimental variables for the control of the particle size. Silica nanoparticles ranged from 10 to 40 nm in average particle diameter were produced by through all the experiments. As the TEOS concentration in the flame increased, the average particle diameter increased. Larger particles were produced with decrease of the total gas flow rate by the variation of air flow rate at 5th tube of the burner. Average particle diameter increased two times larger by replacing Ar with air at the 2nd tube of the burner. As oxygen flow rate decreased, average particle diameter decreased. Particle size showed no change in diameter with small decrease of hydrogen flow rate.