The value of ΔH(Tc ) was estimated on the basis of the difference between strain-rate sensitivity (SRS) at first plateau place and at second one on the stair-like relative shape of SRS and stress decrement due to superposition of oscillatory stress during plastic deformation for four kinds of single crystals (KCl: Mg2+, Ca2+, Sr2+ or Ba2+). ΔH(Tc ) is the enthalpy of activation for the breakaway of a dislocation from the impurity. Tc is the critical temperature at which the effective stress due to only one type of the impurities which lie on the dislocation is zero when the dislocation moves forward with the help of oscillation.