Stress corrosion cracking of A1-5%Mg (5083) alloy was studied in a NaCl+HCl aqueous solution. Intergranular corrosion and stress corrosion cracking are closely correlated with the grain boundary β precipitate and both occured extremely when this precipitate was quite continuous. The crack propagation is discontinuous and the contribution of mechanical rupture is found in the process of cracking. Mechanical rupture, however, plays only a supplemental part. The crack propagation rate can be expressed as an exponential function of the intergranular corrosion rate. The stress corrosion cracking may be essentially a manifestation of intergranular corrosion facilitated by a mechano-chemical effect.