It is well known that prismatic dislocation loops can be formed by the agglomeration of point defects. Molecular statics/dynamics calculations have revealed that extremely small interstitial-type perfect dislocation loops with diameters of less than a few nanometers in metals can be regarded as bundles of crowdions with the same axes (see Ref. [1]). The crowdion bundles can undergo one-dimensional glide diffusion by the almost independent motions of the constituent crowdions along their axes [1]. Also, according to some theoretical studies, the loop diffusion is believed to play important roles in microstructural evolution upon high-energy particle irradiation. In contrast, with increasing loop size, the atomic arrangement in the central region inside loops approaches that of the perfect crystal, and the loops become to be regarded as loops of simple “conventional dislocations” [1]. However, it has remained unclear whether these “conventional dislocation” loops can undergo one-dimensional diffusion because the previous studies on dislocations have hardly dealt with the glide motion of dislocations in the absence of stresses. In this seminar, I will present our results on the motion process of nanometer-sized “conventional dislocation” loops with the Burgers vector of 1/2<111> in bcc Fe with purity of 99.998 weight% upon heating under the application of no external stress and negligible internal stresses, examined by using in-situ transmission electron microscopy [2]. The main results are follows: (i) Loops de-trapped from static impurity atoms can undergo one-dimensional diffusion. (ii) The values of the activation energy for loop diffusion are estimated to be 1.3 eV, independent of the loop size, in the temperature range from 520 to 670 K. (iii) The pre-exponential factor of the loop diffusivity monotonously decreases with increasing loop size. Based on these results, I will propose a simple mechanism of loop diffusion where loops diffuse via the diffusion of each kink composing forward and backward double kinks present at thermal equilibrium. In addition, I will show that the drag of the Cottrell atmosphere of dynamic impurity atoms by loops significantly reduce the loop diffusivity even though the amount of the impurities in the specimen is only a few ppm. References : [1] Y. N. Osetsky, et al. Philos. Mag. 83, 61 (2003). [2] K. Arakawa, et al., Science 318, 956 (2007).