LiAlO2 powder is used as a matrix material in the electrolyte substrate of molten carbonate fuel cells (MCFCs), and LiAlO2 are known to undergo phase transformations between the alpha-phase and gamma-phase in molten Li/K carbonate. In order to understand the phenomena involved in this phase transformation of LiAlO2, it is Clear that it will be necessary to consider the fact that the solubility and mass transfer rate in molten Li/K carbonate differ between these two phases. Also, the solubility differs due to differences in the particle sizes of the LiAlO2 powder, namely, the fine particles dissolve more easily than the large particles. The other factors conventionally considered are the partial pressure of CO2 and the temperature of the surrounding gas. In the present paper, it has become possible that the seemingly complex phenomena of LiAlO2 phase transformation within molten carbonate can be understood without any contradiction,