In-situ SEM observations have revealed that fatigue crack propagation in aluminium alloys is caused by the shear band decohesion around the crack tip and the formation and cracking of the shear band is mainly caused by the plasticity generated in the loading part of the load cycle. This shear band decohesion process has been observed to occur in a continuous way over the time period during the load cycle. Based on this observation, in this study, the transient fatigue crack growth rate, da/dt, has been used to obtain the relationship between the conventional used parameter da/dN and the applied driving force. It is proven that two parameters are necessary in order to accurately describe fatigue crack propagation rate per stress cycle, da/dN. The well known stress ratio effects on fatigue crack propagation rate can be correlated by this model.