The stress corrosion cracking behavior of a commercialized AZ31 magnesium alloy after T4and annealing heat treatments under a vacuum of 1.33Pa has been evaluated via slow strain rate tensile tests (SSRT) in an aqueous 3.5wt.% NaCl solution. The experimental results show that this material displays a high sensitivity to stress corrosion cracking. The fractured surface appears in a transgranular mode. Double-line cracking resulted from twinning can be readily spotted on the cleavage plane. The presence of such deformation twins accompanied by the discontinuous precipitates of Mg17Al12 is the main reason for the stress corrosion cracking susceptibility of this alloy.