The morphology of micro-arc oxidation coatings on AZ91D alloy was analyzed by scanning electron microscopy. The tensile strength and fatigue property of AZ91D magnesium alloy treated with micro-arc oxidation was investigated by static tensile test and fatigue test. And the fatigue fractograph was analyzed by stereomicroscopy and scanning electron microscopy. The results show that the inner layer of micro-arc oxidation coating was imporous and compact, the compact layer had good ductility and good bonding to the substrates. The bonding strength was high. Micro-arc oxidation treatment had no influence upon the tensile strength of AZ91D. The fatigue test condition was as follows, mean value S_m was 4.72 kN, amplitude value S_a was 3.15 kN, stress ratio R was 0.2, load was constant amplitude spectrum and frequency was 10 Hz. The average fatigue life of AZ91D treated by micro-arc oxidation was 54148 cycles, corresponded to that with chemical oxidation.