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.