The objective of this research work was to establish a new rational method for evaluating the whole gelling property of agar substances. Since the advent of the new agar from “alkali-treated Gracilaria” in 1950's, there had been a confusion for qualification when contrasted with the traditional Gelidium agar. Therefore, any rectified scientific method for qualification of gelling property of agar had been generally desired. First, the correlation between the endothermic energy required to dissociate cross linkage of gel, ΔHº, and the organoleptic gel-firmness was found. Secondly, the correlation between ΔHº and ΔTm was also found, whereΔTm is the melting point difference between 1.0 ％ and 1.5 ％ gels. Finally, the new “regression line formula of log ΔHº to log Tm” was established, which extended to the new efficient ΔHº-determination method, by applying the new conventional ΔTm-measurement. It was admitted that the ΔHº implies the intrinsic gelling property of agar and agarose, identically. The ΔHº-range was from 314 to 8158 kJ/mol. The much significance of ΔHº, and practical applications of the results are described in Discussion.