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Upon reversed straining of a pre-strained aluminium at high temperatures, there appears a strain region where plastic deformation proceeds at an approximately constant stress level, and the subgrain structures developed by pre-straining become loosened once and then new subgrain structures characteristic of the reversed straining are formed. In the present experiment polycrystalline aluminium specimens were subjected to a special type of cyclic straining at high temperatures where such a strain region appears.The amount of strain and the stress level in such a strain region and the ratio of the yield stress upon stress reversal to the pre-stress level decreased with the progress of the cyclic straining. These facts suggest that the stability of dislocation structures upon stress reversal gradually increases, that is, the tendency to undergo changes due to the stress reversal is reduced as the cyclic straining progresses. Transmission electron microscopy showed that the increase in the stability of dislocation structures is caused by the decrease in density of isolated dislocations and also by the change of sub-boundaries from an unstable ill-defined state to a stable well-defined one.