Studies on sodium aerosol characteristics are very important for fast reactor safety. Physical and chemical changes that occur at various times at various atmospheric conditions to sodium aerosols would results particles in various diameters. In this context, a study has been conducted in Aerosol Test Facility, Radiological Safety Division, in which sodium combustion aerosols were generated in a controlled manner and made to hover in a confined volume. The particle diameter (Mass Median Diameter - MMD) is measured on-line with progress of time by using Mastersizer. The experiments are conducted by generating aerosols in two different mass concentrations viz. 3.0 g m–3 and 0.5 g m–3 and in three different relative humidity conditions viz. 20%, 50% and 90% to show the influence of relative humidity on the particle diameter. A theoretical simulation of particle growth due to coagulation is drawn and compared with the experimental value. It is observed that sodium combustion aerosols size grow due to absorption of moisture (hygroscopic growth) in the initial period of times say in first 20 minutes followed by Brownian coagulation. An empirical relation is determined based on the difference between coagulation growth and experimental observation and applied to match experimental observation of particle diameter. A detailed experimental procedure, theoretical simulation and comparison of results of particle growth between experimental observation and theoretical simulation are presented in this paper.