The thermo-physical properties of the phase-change latent heat storage material sodium acetate trihydrate were elucidated in order to examine its temperature dependency and supercooling phenomenon. Heat transfer by natural convection was also investigated in a horizontal enclosed rectangular container filled with sodium acetate trihydrate which was heated at the lower surface heating and cooled at the upper surface. The supercooling of sodium acetate trihydrate was clearly influenced by the cooling rate, and two kinds of supercooling phenomena were found. It was also 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 thermo-physical properties obtained in the present work.