It has been previously reported for KCl : Sr2+ (0.035, 0.050, 0.065 mol% in the melt) single crystals that the interaction between a dislocation and the impurity can be approximated to the Fleischer's model. From the values of phi(0) for the specimens, however, it was confirmed that the Friedel relation can be taken into the Fleischer's model. The phi(0) is the bending angle at which the dislocation embraces the impurities under the effective shear stress at 0 K. Furthermore, the interaction between a dislocation and the impurity could be approximated to the Fleischer's model taking account of the Friedel relation. This was examined on the basis of the dependence of strain-rate sensitivity due to the impurities on temperature at about 100-200 K. Then, the critical temperature, at which the effective shear stress is zero, was determined to be 282 K. In addition, the values of the enthalpy and the Gibbs free energy of activation for the breakaway of the dislocation from the impurity were obtained for the specimen.