Ethanol is a familiar substance to everybody, but its water solution is a mysterious substance in that it shows complex changes in many of its properties depending on ethanol concentration. Ethanol water mixture has a liquidus line and a solidus line that one separated, and therefore it can have both liquid and solid phases existing together. As result, with advances in low temperature technology in recent days, ethanol water mixture is attracting more and more attention as an environment-friendly coolant or as a thermal storage material. In the present work, we developed the fluid latent heat storage material using ethanol water mixture.