Effects of grain boundary segregation on fatigue crack growth in 7050 aluminium alloy have been investigated under various aging conditions in a humid air environment. The results show that with increased aging, the concentration of Mg segregation on the grain boundary decreases, resulting in a reduction in the fatigue crack growth rate for the same humidity and value of Δ K. The effect of grain boundary Mg segregation on the fracture stress has been calculated using the quasi-chemical method. The result indicates that Mg segregation induces fracture reduction and grain boundary embrittlement.