Extensive simulations based on classical molecular dynamics have shown that small interstitial-type perfect dislocation loops in various metals and alloys have the structure of bundles of crowdions and a loop can easily makes the one-dimensional glide motion due to almost independent motion of crowdions in the loop. However, the experimental knowledge on the motion of loops is not enough. The present study dynamically examined the motion process of loops in pure iron under 1000 keV electron irradiation and thermal annealing by using transmission electron microscopy under which loops could move. Two types of loops were formed by irradiation. Loops of one type possessed the Burgers vector of 1/2 < 111 > and the habit plane of {011}, and loops of the other type were < 001 > {001}. Loops of the former type made back-and-forth glide motion and expansion towards the direction along their Burgers vectors when they were smaller than about a few-ten nanometers in diameter. This strongly suggests that these small 1/2 < 111 > loops have the structure of the bundle of crowdions. Loops of the latter type only rarely moved less frequently when they were smaller than about the same size. When loops of two types grew larger than about 50 nm, the characteristics of the motion of loops changed drastically. Dislocation segments of each large loop made long-distance glide independently of their opposite segments, and the habit plane deviated from the original ones. This kind of motion means that self-interstitial atoms at the central region of such large loops are no longer the crowdions.