An experimental method has been developed to measure fatigue weaklink density and strength distribution in high performance Al alloys, such as AA2016, AA2099, AA2524 and AA2024 Al alloys, and cast 713 Al-Zn alloy, etc. In this method, fatigue crack population is measured at different cyclic stress levels. The crack population is a Weibull function of the applied stress, which could be used to determine fatigue weaklink density and strength distribution in these alloys. Fatigue weaklink density and strength distribution are materials fatigue properties useful for evaluating the fatigue behaviors of materials. The anisotropy of fatigue crack initiation and environmental effects on fatigue crack initiation could be studied explicitly by studying these properties in high strength Al alloys. This method could not be applied to those alloys that are brittle or low in strength or have low tendency for planar slip, since they do not show a multiple crack initiation behavior.