In this study the corrosion resistance in 5 % NaCl electrolyte and detailed examinations of microstructure of magnesium alloys in as cast, T4 and T6 conditions were carried out. The microstructure examinations were conducted using scanning electron microscope and transmission electron microscope. In addition, the oxide films formed on the surfaces of the magnesium alloys, exposed to air and immersed in 5 % NaCl solution, have been investigated by the surface sensitive auger electron spectroscopy. The aim of this paper was to understand the strongly effect of microstructure and second phases on the corrosion behavior of magnesium alloys. The paper will emphasise the role of Mg17Al12 and Al-Mn-Fe phases on corrosion resistance of magnesium alloys.