The effect of the shape of load exceedance distribution on fatigue crack growth behaviour under random amplitude loadings has beeen investigated. Fatigue crack growth tests were conducted under load spectra which were derived from a two-Darameter Weibull distribution, Q ( ΔP /σ)=exp (- k ( ΔP /σ)^1/k) Q : Probability of ΔP exceeding σ ΔP : Load range σ, k : Weibull parameters Three kinds of spectra, i. e. convex, linear and concave shapes in load-log exceedance curve were generated choosing the Weibull parameter, k , in the above equation as 1/2, 1 and 2 respectively. The fatigue crack growth behaviour under the spectra was discussed in terms of equivalent stress intensity factor, ΔK _eq, which was calculated from a linear cumulative damage law. The effect of the shape of the load exceedance distribution on the fatigue crack growth was significant in order of the concave, the linear and the convex shape in low ΔK _eq region. In this region, the fatigue cracks under the random loadings propagated faster than those under constant amplitude loading. Even in the region where the ΔK _eq, was smaller than ΔK _th obtained from the constant amplitude test, fatigue crack growth was observed under the load spectra. On the other hand, the difference in crack growth among the spectra was not found in intermediate ΔK _eq region. The fatigue crack growth retardation was observed in this region. The relationship between crack growth rate and stress intensity factor range under the random amplitude loadings was examined for various orders of equivalent stress intensity factor, such as root-mean-square, root-mean-cube values. It is concluded that the fatigue propagation lives under the random amplitude loadings can be conservatively estimated in terms of the equivalent stress intensity factor which corresponds to the slope of log da/dN- log ΔK relationship obtained from the constant amplitude test, if the ΔK _th is ignored.