Thermo-physical properties of the phase-change latent heat storage material sodium sulfate decahydrate were elucidated in order to examine its temperature dependency. Heat transfer by natural convection was also investigated in a horizontal enclosed rectangular container filled with sodium sulfate decahydrate, which was heated at the lower surface and cooled at the upper surface. It was found that the heat transfer coefficient of the liquid phase coincides well with the conventional heat-transfer correlation of natural convection heat transfer in a horizontal enclosed rectangular container by applying the values of thermophysical properties obtained in the present work.