Abstract: Experimental research on the heat conductivity has been carried out, focused on the influence of the magnitude and density of CaCl 2 particles as a thermal adsorption refrigeration material. The system of measurement for the heat conduction coefficient is built upon the principle of concentric cylindric walls. The heat conductivity of CaCl 2, which is well-distributed, is studied by experiment. The regression equations and the curves of heat conductivity are obtained, revealing their changes with the grain size. The results show that there exists a critical magnitude in the grain size on the heat conductivity of CaCl 2, and that the critical magnitude has nothing to do with the density of CaCl 2. Proceeding from the principle of heat transmission, the factors of influence upon the heat conduction coefficient is explained. Also a new theory on the critical size is brought forth.