The effects of evaporation rate and pressure of helium gas on the size of ultrafine tin oxide powder were studied in a gas condensation method. The velocity and size of convection current of helium gas were regulated by varying gas pressure in the chamber and temperature of an evaporation source. As the convection current became slow in velocity and reduced in size, the average size of the synthesized tin oxide powder increased. The respective effects of evaporation rate and gas pressure were analyzed by computer simulation of convection currents at a steady state condition, based on the experimental results. Oxygen gas flown into a condensation chamber filled with constant helium gas provided a proper condition for synthesis of ultrafine tin oxide powder having a narrow size distribution.