A strain-rate cycling test during the Blaha effect measurement was carried out at 83-239 K for the purpose of studying the force-distance relation between a dislocation and the aggregate for KCl : Sr2+ (0.05 mol% in the melt) single crystals. On the basis of the dependence of strain-rate sensitivity due to the aggregates on temperature, it was found that the interaction between a dislocation and the aggregate in the specimen can not be approximated to the Fleischer's model taking account of the Friedel relation within the temperature. The square force-distance relation between a dislocation and an impurity seemed to be the most suitable model among the three: a square force-distance relation, a parabolic one and a triangular one, taking account of the Friedel relation for the specimen. In addition, the values of enthalpy and Gibbs free energy of activation for overcoming of the aggregate by a dislocation were obtained for the three force-distance relations.