Abstract : Thermal batteries are primary batteries that specially designed to a high energy density in a short period. The unit cell of thermal batteries use molten salts as an electrolyte and employ an internal pyrotechnic source to heat the cell stack to operating temperatures, typically between 400 and 600 oC, heat transfer is crucial to the operation of the battery, determining the operation time in which power can be supplied. Thermal analysis is a critical part of thermal battery design because of the need to hold the temperature above the electrolyte melting point. This paper mainly focused on the transient thermal model studies of unit cell to simulate the heat transfer of unit cell. Thermal properties in the electrolyte caused by phase change are a critical design consideration of thermal batteries because the thermal conductivity and specific heat could be changed. The simulation results show the trends in temperature and heat flux.