An attempt to select the most suitable force-distance relation between a dislocation and the monovalent cation impurity in KCl:Li+ (LiCl 0.5 mol% in the melt) and KCl:Na+ (NaCl 0.5 mol% in the melt) was conducted among the three models: a square, a parabolic, and a triangular force-distance relations. These models are taken into account the Friedel relation. The grope for the most suitable model was carried out at 80-300 K by roportionality of ∆H(T) . The ∆H(T) is the activation enthalpy for the interaction between a dislocation and an impurity as a function of temperature. As a result, the square force-distance relation seemed to be the most suitable of the three for KCl:Na+. However, it was difficult to select that for KCl:Li+. ∆H (Tc) was further obtained on the three force-distance relations for both the pecimens. ∆H (Tc) corresponds to the activation enthalpy for overcoming the strain field with cubic distortion around the substitutional impurity by a dislocation without an applied stress.