This criterion is based on the occurrence of an extremum of hot spot parameters with respect to any of the input parameters of the reactor. The critical operation trajectory, the critical isoconcentration curve and the supercritical operation trajectory are defined. Applying the relationship between the hot spot of the critical operation trajectory and the inflection point of the supercritical, an approximate expression of the critical inlet conditions is derived. The computed results show that the influence of the cooling medium on the parametric sensitivity region is related to the heat capacity ratio alpha. And the greater alpha is, the more sensitive the reaction systems would be.