Because most radioactive materials that can escape from a nuclear power plant during a severe accident are expected to be in the form of aerosols, the installation of a filtered containment venting system (FCVS) will be effective to mitigate the risks caused by radioactive aerosols. Aerosol size is a parameter important to the design requirements of an FCVS because the collection efficiency of the venting system depends on the size of the aerosol. In this study, the size distribution change of aerosols by condensation was calculated by using the moment method. Sodium chloride was used as nuclei that underwent condensational growth, and Di-Ethyl-Hexyl-Sebacate (DEHS) was used as a vapor that participated in condensational growth. Then, a condensation experiment was conducted to verify the results calculated by the moment method. However, in an actual severe accident, water vapor in the containment would condense on particles. Therefore, after model verification, calculation was performed with water vapor as the condensation vapor to predict the condensation scenario under a severe accident. This paper reports that the aerosol condensation model based on the moment method can be an auxiliary tool in an existing aerosol modeling program to estimate the particle size distribution change during a severe accident.