Propagation behaviour of fatigue cracks in aluminium was examined at room temperature, 201゜K,and 78゜K. The purpose of the present investigation is to make clear the fatigue mechanism and also to confirm whether or not the vacancy absorption model for the growth of fatigue cracks, which has been proposed by the present authors, is valid in the case of low temperature fatigue as well. Slip-line-concentrating region around a fatigue crack was found to be considerably smaller at 201゜K and 78゜K than

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... at room temperature. It was also clarified that a power law, dl/dN=C1KnI, well-known at room temperature, also holds between the rate of fatigue crack growth, dl/dN, and the stress intensity factor, KI. In the temperature range examined the growth rate was remarkably affected by temperature, which is expressed by an Arrhenius type equation, dl/dN=C2KnIexp(-Q/kT) where Q=0.03-0.04eV. These results suggest that it is important to take into account a thermally activated process in considering the mechanism of fatigue crack growth. In conclusion, it is shown that the strong temperature dependence of the growth rate can satisfactorily be explained by the vacancy absorption model, provided that the diffusion of vacancies along dislocation cores is assumed.