The interaction between a dislocation and the impurity in KCl:Sr2+ (0.035, 0.050, 0.065 mol% in the melt) single crystals can be approximated to the Fleischer’s model taking account of the Friedel relation. This is because the bending angle, φ0 , at which the dislocation embraces the impurities is within the range of 149° to 161° for the specimens under the effective stress at the temperature of 0 K. Then, the critical temperature, c T , at which the effective stress is zero was determined to be 289 K from the linear plots of (τp1/ τp0)1/ 3 and (temperature)1/2 for the specimens. τp0 is the effective shear stress τp1 due to impurities without thermal activation.