The fatigue crack propagation (FCP) behavior of magnesium alloy AZ61 at room temperature (RT), elevated temperature (60°C, 120°C) , and in ambient and wet air was investimated. The mechanisms of FCP were discussed in detail. It was demonstrated that The FCP rate of AZ61 magnesium alloy increased with increasing temperature and relative humidity (RH). Obvious change in the microstructure occurred during fatigue at elevated temperature, particularly at 120°C, compared to its original microstructure. Grain growth, deformation twin, grain boundary (GB) immigration and precipitates were observed in the microstructure at 120°C after fatigue. A bend occurred in the curves of FCP rate versus stress intensity factor at 120°C, which corresponded to a transition of failure mode from a mixed intergranular and transgranular fracture to a transgranular fracture. At first stage, the FCP rate increased sharply, and then went up slowly due to the growth of grain size. Secondary phase particles facilitated the fatigue crack initiation. The Hydrogen embrittlement (HE) may be primarily responsible for accelerating FCP rate in wet air.