Torsional fatigue experiments were carried out with Al-4% Cu specimens at various aging stages in order to study further the dislocation pinning process occurring under fatigue loading. The area and the shape of the hysteresis loop after various stress cycles N were determined, from which the energy loss AE in each cycle and the maximum forque Tm of the specimen were calculated.The temperature and the time of aging were so chosen that G. P. [1] zone, G. P. [2] zone, θ′ phase or the stable θ phase appeared respectively in one of the specimens. On the basis of comparing the mode of changes of the experimental curves of △E-N and Tm-N, it is concluded that the dislocation pinning occurring in the initial stage of fatigue loading is not concerned with the transformation products such as the G. P. [1] zone and G. P. [2] zone.The changes of the energy loss in the first cycle of fatigue loading, (△E)1, of specimens at various aging stages were compared and analysed, and it is shown that, in the case of the Al-Cu alloy studied, the energy loss △E is produced because of the occurrence of a fluctuating internal stress field in the lattice near a dislocation, since work has to be done when the dislocation moves to and fro in such a stress field. This fluctuating internal stress field may appear because of the occurrence of point defects (vacancies and solute atoms), atom clusters, G. P. [1] zone, G. P. [2] zone, or other defects when they do not segregate or nucleate on the dislocation line.On the basis of the above analysis, it may be considered that the dislocation pinning process occurring under fatigue loading is associated with the formation of atmospheres of solute atoms. The solute atoms migrate to dislocations in the process of fatigue loading with the help of the vacancies produced in fatigue loading, so that atmospheres are formed around dislocations. The mobility of these pinned dislocations is reduced, and consequently △E is lowered. The θ′ and θ phases that nucleate on dislocation lines during the process of aging can also pin the dislocations to a certain extent.It can also be contemplated from the above picture that the G. P. [1] and G. P. [2] zones do not nucleate on a dislocation line, while the θ′ and θ phases do.