The pure and fine silica particles are widely used in the semiconductor industries. In our previous paper, the authors have demonstrated that production of title silica powder is possible by gas phase hydrolysis of tetramethoxysilane (TEMS). In this paper, we report the effect of the shape of reactor and operational condition, such as mixing condition and temperature on conversion of the reaction and properties especially particle diameter of produced silica fine particles. The different shapes of reactors named as Type 1, Type 2, and Type 3, were tested. In Type 1, H2O and TEMS were introduced cocurrently and heated in the reactor. On the other hand, in Types 2 and 3, TEMS introduced from the top was vigorously mixed, at relatively upper and rather vicinity of the center parts respectively, with heated H2O introduced from the bottom through the center part of the reactor. Thus TEMS is mixed with more heated water in case of Types 2 and 3 than Type 1. Type 3 has a shorter water introduction pipe and TEMS and H2O are mixed at higher temperature than Type 2. The average diameters were decreased as the order of Type 1, Type 2 and Type 3. It was demonstrated that smaller fine particles were produced by rapid heating of TEMS under coexistence of water vapor. Under the condition, only nucleation reaction is expected to be accelerated, by Suppressing particle growth through heterogeneous reaction. The mixture condition of the gas leading to the heating condition of TEMS with water vapor Could be changed by changing the shape of reactor, which resulted in the change in averaged diameter of the produced silica particles.