Abstract: The effects of secondary ageing of T7951 on fatigue crack propagation behavior of 7055 aluminum alloy at room temperature in the air were investigated through the crack propagation rate test and SEM observation. Results show that the fatigue crack propagation rate of the alloy decreased after secondary ageing. The stable crack propagation areas in fatigue fracture of the aluminum alloy before and after secondary ageing all exhibited quasi-cleavage fracture, and parallel fatigue striations were observed in the above areas. Secondard cracks occured in the aluminum alloy before secondary ageing, and the distance between fatigue striations before secondary ageing was bigger than that after secondary ageing. Transgranular dimple fracture were observed in the final fracture areas in fatigue fracture of the aluminum alloy before and after secondary ageing.