The fatigue crack propagation behavior of AZ91HP alloy was observed and the fatigue fracture structure as well as crack propagation mechanism was described in the condition of as die-cast（F）

solid solution treatment（T4）and solid solution plus aging（T6）treatment.The results reveal that the fatigue crack propagation behavior of the alloy is strongly dependent on the value of stress intensity factor（△K）

in which with decreasing △K

the fatigue fracture becomes more and more smooth

and in turn with the increase of △K

the fracture becomes rougher.The plasticity-induced crack closeness effects were observed in the process of fatigue crack propagation

which can result in the increase of the value of crack-opening stress intensity factor（Kop）and the decrease of effective stress intensity factor（△Keff）.The crack propagation rate was inversely proportion to the plasticity of AZ91HP alloy.Because the plasticity of the alloy was increased in T6

F

T4 sequential order

the crack propagation rate was decreased in T6

F

T4 sequential order.