In this paper, the fatigue crack propagation (FCP) behaviors of the forged Mg-Zn-Y-Zr alloy with different tempers were investigated. At the stage 1 of the FCP, the K-da/dN curves were distinguished from each other. However, at the stage 2 of the FCP, the K-da/dN curves were almost the same. SEM fracture analysis indicated that at the stage 2 of FCP, the fracture characteristics of the forged and T5 samples were similar, with many deep dimples on the fracture surfaces, whereas the fracture surfaces of the T4 and T6 samples were covered with lots of lamellar cleavage planes. SEM observations also indicated that the microstructures were distinguished from each other. For the forged and T5 samples, many big bulk w-Mg3Y2Zn3 phases distributed in the Mg matrix. However, for the T4 and T6 samples, due to the solid solution treatment, w-Mg3Y2Zn3 phases were almost completely decomposed and many coarse rod-like 1 precipitates appeared in the interior of the grains. These firmly confirmed that the variation of the microstructures had great influence on the fatigue crack propagation (FCP) characteristics of the Mg-Zn-Y-Zr alloy.