A secondary refrigerant can be produced by using a multi-component material-such as a solution of some material in water, where there exists a liquid-solid co-existent region. Using this solid-liquid co-existent region, if we can form a two-phase solid-liquid flow, a high performance secondary refrigerant can be obtained. In addition, if the components of the multi-components material are existent in the nature, the secondary refrigerant can be used as a natural secondary refrigerant. In the present study, possibility of forming a new secondary refrigerant was investigated using a urea-water mixture. We made visual observations on crystal growth in a urea-water mixture, and using this information, we developed a new fluid latent heat storage material. And the differential scanning calorimeter (DSC) was used to investigate the phase diagram and the latent heat of a urea-water mixture. In addition, based on this phase diagram, the experiments were conducted under stirred conditions and fluid latent hest storage materials were investigated for various concentrations of urea-water mixtures.