PURPOSE: To obtain a nucleating agent for preventing supercooling of a latent heat thermal energy storing material, by adding a nucleating agent to the latent heat thermal storing material, heating, cooling and solidifying the resultant blend, reheating the blend and filtering off the aimed agent from the melt.

CONSTITUTION: A nucleating agent, such as trisodium phosphate.dodecahydrate, in an amount of about 1W40wt.% is added to a latent heat thermal energy storing material, such as sodium acetate.trihydrate, and the resultant blend is heated at about 85W110°C and then cooled to once solidify the thermal energy storing material. The obtained blend is then reheated at the melting temperature of the above-mentioned thermal energy storing material or above and the heating limit temperature or below (e.g. about 60W90°C) and subsequently filtered still in the molten state to afford the aimed nucleating agent capable of effectively preventing supercooling simply by adding about 0.1W5wt.% thereof to the latent heat thermal energy storing material.