Dynamic mechanical properties of AZ31 magnesium alloy plate were carried out using split Hopkinson pressure bar (SHPB) with compression direction 0° and 90° from normal direction respectively. Optical microscopy (OM) and scanning electronic microscopy (SEM) were used for the observation of microstructure and fractograph. OM observations showed that cracks initiated and propagated along localized deformed bands consisted of twin intersection for 0° specimen, and that cracks initiated and propagated along localized transformed bands consisted of fine equiaxed dynamic recrystallized grains for 90° specimen. SEM observations showed that the fractograph exhibited typical ductile dimple fracture pattern for 0° and 90° specimens due to the thermal accumulation in localized bands caused by severe plastic deformation. The fracture mechanisms are anisotropic for AZ31 magnesium alloy plate, which contributed to the dynamic mechanical anisotropy of AZ31 magnesium alloy plate.