The process of synthesizing silica nanopowder by the gas phase thermal oxidation of tetraethylorthosilicate in a diffusion flame reactor was simulated using a commercial computational fluid dynamic code. The fuel combustion process and silica particle formation and growth in the flame were modeled. The temperature, velocity, and particle size distribution (PSD) fields inside the reactor were computed. Chemical reaction rate and a population balance model were used to calculate the particle formation and growth and PSD. Satisfactory results were obtained for the temperature profile and PSD. Computed values of the average size of the final products were consistent with those obtained in a previous experimental work.