Torsional fatigue experiments were carried out on 99.6% commercial aluminum and 99.99% high-purity aluminum (annealed and cold-worked). The shape and the area of the hysteresis loop (stress versus strain) after various stress cycles N were determined, from which the energy loss△E in each cycle and the maximum torque Tm of the specimen were calculated. The △E-N curves and Tm-N curves were analyzed and different stages of the curves were correlated with the results of metallographic observations on the specimen surface during the process of fatigue. It is then concluded that, in the initial stage of fatigue loading, the basic process that gives rise to the change of △E and Tm is the pinning of dislocations by the vacancies generated during fatigue and this occurs as a bulk effect. Later on, the localized coarse slip regions appearing in the specimen give rise to an extra energy loss △E, but have only an insignificant effect on Tm. Consequently, the changes of Tm and △E in this stage no longer correspond to each other. Such an analysis may clarify many of the dis-crepencies reported in the literature concerning the results of observation on the change of △E and Tm (representing a change in hardness) during fatigue.Assuming the curves experimentally observed to be a summation of the effects given rise by these two processes (a bulk process and a localized process), and taking into account that the contributions from these two processes on △E and Tm vary with the fatigue amplitude, the condition of specimen treatment (annealed or cold-worked), and the impurity content of the specimen, the shape and the position of the numerous experimental curves of △E-N and Tm-N can be interpreted satisfactorily. However, the validity of this viewpoint needs further experimental verification.