A study was made on the effect of various treatments upon the energy loss △E and the maximum torque Tm in aluminum-copper alloys containing 1%, 2% and 4% copper under fatigue loading. Metallographic observations were made correspondingly.Experiments made with Al-1% Cu alloy showed that when the applied torsional strain is large, the shapes of the △E and Tm curves are similar to that of high-purity aluminum and commercial aluminum. However, when the torsional strain is small, or when the specimen was aged at room temperature before the fatigue experiment, the shapes of the △E and Tm curves are similar to those of Al-4% Cu alloy. These results can all be interpreted in terms of the interaction between solute atoms and dislocations.Experiments on intermittent loading were made on Al-1% Cu and Al-2% Cu alloys. The results confirmed the viewpoint that the up-rise of △E in the later stage of fatigue experiment are due to the occurrence of coarse slip bands in the specimen, and is not due to the softening of the specimen associated with over-aging.Re-solution treatments were made in fatigue experiments of Al-4% Cu alloy. Judging from the behavior exhibited by the △E and Tm curves after each treatment, the conclusion may be drawn that the fatigued specimen can be made to recover to its original condition if the specimen is treated immediately after the up-rise of △E. However, if the specimen is treated a while after the up-rise of △E, the fracture of the specimen is sped up because of the occurrence of fatigue cracks in the specimen.Two conclusions having practical significance can be made on the basis of our experimental results: (1) It is possible to detect the formation of initial fatigue cracks through the method of measuring △E, because the occurrence of coarse slip bands is a prelude of crack formation; (2) it is possible to delay the occurrence of coarse slip bands and thus to raise the fatigue life through the method of alloying and heat treatment.