Strain-rate cycling tests associated with ultrasonic oscillation were carried out at 80-239 K for two kinds of KCl:Sr2+ ( 0.05 mol.% in the melt) single crystals: one is a quenched specimen and the other an annealed one. In this study, it was found that the density of moving dislocation is not influenced by the heat treatment. Furthermore, the increase in forest dislocation density for the annealed specimen seemed to be remarkable under the compression test, compared with that for the quenched specimen. As a result, the strain-hardening rate increased and the extent of plastic deformation region became short at a given temperature by annealing the quenched specimens. The investigation concerning forest dislocation density was conducted on the basis of the Delta(Delta tau '/Delta ln(epsilon) over dot)/Delta epsilon, which will represent the variation of the strain-rate sensitivity due to dislocation cuttings with shear strain.