Source term for severe accident analysis of molten salt reactors(MSRs) has been investigated as part of preliminary efforts to develop MSRs. As a severe accident of MSRs, exposure of heated fluoride fuel molten salt to atmosphere was assumed to take place. Vaporization of fluoride molten salt was studied by means of the two methods, the Knudsen effusion mass spectrometry as well as the transpiration method. The former was applied to pseudo-binary fluoride systems to clarify the behaviors of cesium and iodine in the fluoride molten salt. The latter was applied to the mixture of CsI and FLiNaK. These experiments were carried out as the first step of the source term studies, so that interaction with air components has not been covered yet. From this study, useful information related to the source term for MSRs have been obtained. This work suggests how to solve the problem to establish the source term for severe accident analysis of MSRs.