Abstract: This paper mainly deals with the relation between the strain fatigue properties and the fatigue dislocation structures in 2024 aluminum alloy. Some interesting features of the fatigue dislocation structure in this commercial alloy are observed.The analysis of deformation work density for smooth fatigue specimens shows that the strain fatigue properties of specimens aged at 240℃ are . much better than naturally aged specimens at high strain level.Systematic study of fatigue dislocation structures in this alloy under the transmission electron microscope reveals that wavy slip mode at high strain level and plain slip mode at low strain level are characteristics of the dislocation slip in the naturally aged specimens, in contrast cross slip is much easier in the 240℃ aged specimens at different strain levels. By foil thickness analysis the existence of {112} slip plane in 240℃ aged specimens was confirmed.It is also pointed out that the helical dislocations in the alloy are formed through cutting of moving dislocations with dislocation loops.The conclusion has been drawn that the features of dislocation slip serve as an essential factor which affects the interaction of dislocations with other microstructures and determines the fatigue properties.