The interaction between a dislocation and the impurity in four kinds of single crystals (KCl: Mg2+, Ca2+, Sr2+ or Ba2+) was investigated on the basis of τp1 at 77－254K.τp1 is considered to represent the effective stress due to only one type of the impurities which lie on the dislocation when the dislocation moves forward with the help of oscillation, as reported in this series. As a result, it was found that the critical temperature, at which τp1 is zero, tends to increase when the divalent ionic size is close to that of K+. The bending angle of dislocation by the ion was furthermore obtained for the specimens.