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degree of the substructure formation was not uniform over a specimen in the transient stage so that the substructures were different from grain to grain, while it was unifom in the secondary stage. The subgrain size, the dislocation density within subgrains, and the dislocation spacing in sub-boundaries were dependent not on creep temperature but on creep strain and stress. The dislocation spacing in sub-boundaries determined in the present work showed the stress dependence similar to that of the elementary jog spacing expected from a super-jog model which has recently been proposed by two of the present authors for high-temperature creep of alpha-iron.Many dislocation loops, the density of which depended on the creep condition, were observed within subgrains. The dislocation loops of large size were analysed to be of the vacancy-type.