Recently, magnesium (Mg) alloys have become of major interest as a light-weight structural material because of their excellent specific strength. In extensive applications for structural components, fatigue properties such as fatigue strength and fatigue crack propagation (FCP) are critical, but studies on FCP behaviour are very limited. Kamakura
] have indicated that FCP behaviour in wrought Mg alloys was influenced significantly by humidity in laboratory air. This is due to very high electro-chemical activation resulting from poor corrosion resistance (Eliezer
]). Therefore, it is necessary to further study the FCP behaviour and mechanisms in corrosive environments. In the present paper, FCP tests have been performed using CT specimens of AZ31 and AZ61 in dry air, in laboratory air and in distilled water, and the effect of environment and mechanisms were discussed.