We have proposed a simplified model equations for a diffusion flame reactor to prepare ultrafine TiO₂ powders, based on the assumption of plug flow. The model equations such as mass balance of TiC1₄, the 0th, 1st, and 2nd moment balances of aerosol were solved by a numerical simulation. The particle characteristics such as the concentrations, sizes and standard deviation of TiO₂ particles were theoretically investigated along the reactor length, varying the process conditions of flame temperatures, total gas flow rates and inlet reactant concentrations. It is found that the TiO₂ particle concentration at the reactor exit becomes higher, as the total gas flow rate increases, and also as the flame temperature decreases. On the other hand, the TiO₂ particle size increases, as the flame temperature and the inlet TiC1₄ concentration increase and as the total gas flow rate decreases.