Cold crystallization is an exothermic, monotropic transition that occurs at a temperature below the sample’s normal melting point during the heating process. It is thus possible to utilize the phenomenon so that a heat storage material can eliminate the time lag between heat supply and demand. In an attempt to examine some metal complexes as potential candidates for heat storage materials, Schiff-base?Ni complexes bearing tolyl (OT, MT, and PT) and isopropyl (IP) groups at the Nterminal were prepared and investigated by thermal analysis, X-ray diffractometry, FTIR-DSC measurement, and DFT calculation. The behavior of supercooling and cold crystallization observed for these four compounds led to the following two basic guidelines for designing materials to exhibit the desired heat storage property: (1) the molecular flexibility should be enhanced in order to increase the activated energy for crystallization; (2) the liquid state consists of a mixture of two (or more) conformational isomers. These guidelines are useful to prevent the molecules from crystallization during cooling, and to afford a glassy state via deep supercooling, resulting in cold crystallization in the subsequent heating process.