The behavior of sodium hydroxide aerosol is one of the important features in the safety evaluation of a sodium-cooled fast reactor. The vapor phase combustion of the sodium results in the generation of sodium oxide aerosols in the flame zone. The physical and chemical changes of sodium oxide aerosol after leaving the flame zone are very important to understand the sodium oxide aerosol process. The particle size distribution of sodium hydroxide aerosol after leaving the flame zone is measured using both optical and impaction techniques in the Aerosol Test Facility. The mass median diameter is found to be ~1.0 m for the particles older than 20 s. The size distribution is essentially attributed to sodium hydroxide aerosol. The initial size distribution of particles does not vary with ignition temperatures over the range of 250 to 550°C for a confined pool, whereas the size increases with increase in relative humidity.