This study investigates the effect of gas flow rates of air, nitrogen and argon on the synthesis of silica nano-powders in a diffusion flame reactor. The silica powders were prepared by high temperature oxidation of tetraethyl-ortho silicate (TEOS) in a three-port diffusion flame burner. The present configuration resulted in synthesis of silica powders in the range of 45 to 69 nm. The characteristics of the burner design in terms of flame height and temperature have been extensively investigated over a wide range of flow conditions of the fuel (H-2), oxidizer (air), and the carrier gas (N-2) and correlated to the resultant silica powder size. The flame height decreases with increase in air and nitrogen flow rate. It is found out that the fuel-air mixing plays an important role in controlling the particle size as it affects the flame temperature. Increasing the flow rate of nitrogen or air increases the silica size while an increase in fuel flow rate results in slight reduction in the particle size. (C) 2008 Elsevier B.V. All rights reserved.