The interaction between a dislocation and the impurity in KCl:Mg2+ (0.035 mol% in the melt) was investigated at 77-178 K with respect to the two models: one is the Fleischer's model and the other the Fleischer's model taking account of the Friedel relation. The latter is termed the F-F. The dependence of strain-rate sensitivity due to the impurities on temperature for the specimen was appropriate to the Fleischer's model than the F-F. Furthermore, the activation enthalpy, DeltaH, for the Fleischer's model appeared to be nearly proportional to the temperature in comparison with the F-F. The Friedel relation between effective stress and average length of the dislocation segments is exact for most weak obstacles to dislocation motion. However, above-mentioned results mean that the Friedel relation is not suitable for the interaction between a dislocation and the impurity in the specimen. Then, the value of DeltaH(T-c) at the Fleischer's model was found to be 0.61 eV. DeltaH(T-c) corresponds to the activation enthalpy for overcoming of the strain field around the impurity by a dislocation at 0 K. In addition, the Gibbs free energy, DeltaG(0), concerning the dislocation motion was determined to be between 0.42 and 0.48 eV on the basis of the following equation partial derivativeIn(epsilon) over dot/partial derivativetau = {DeltaG(0)/(kT tau(p0))}{1 - (T/T-c)(1/2)}(-1)(T/T-c)(1/2) + partial derivativeIn (epsilon) over dot/partial derivativetau where k is the Boltzmann's constant, T the temperature, Tc the critical temperature at which the effective stress due to the impurities is zero, tau(p0) the effective shear stress without thermal activation, and. (epsilon) over dot(0) the frequency factor. (C) 2003 Kluwer Academic Publishers.