The interaction between a dislocation and the impurity in KCl : Mg2+ (0.035 mol% in the melt), KCl : Ca2+ (0.035 and 0.065 mol% in the melt) and KCl : Ba2+ (0.050 and 0.065 mol% in the melt) was investigated from the strain-rate cycling test during the Blaha effect measurement. This was carried out at 77-254 K. As a result, it was found that the Fleischer's model taking account of the Friedel relation seems to be suitable for KCl : Ca2+ and KCl : Ba2+. However, it was not appropriate for KCl : Mg2+. Furthermore, the values of T-c, DeltaH(T-c) and DeltaG(0) were obtained for the specimens. T-c is the critical temperature at which effective stress is zero. DeltaH(T-c) and DeltaG(0) are the enthalpy and the Gibbs free energy of activation for the breakaway of the dislocation from the impurity, respectively. DeltaH(T-c) was almost the same for the specimens except KCl : Mg2+. DeltaG(0) increased with increasing the divalent cation size. In addition, the tetragonality around the divalent ion-positive ion vacancy pair was estimated on the basis of DeltaG(0) for the each specimen.