Mg-7.3Al magnesium alloys were investigated in the paper. The paper presents microstructural characterization of Mg-7.3Al alloy after casting and heat treatment. The casting temperature was 740°C and heat treatment was performed at 420 °C for 24 h with aging at 180 °C for 8h,16h,24h. The microstructure of the casting alloy consists of α-Mg phase matrix with a primary β phase (Mg17Al12) at grain boundaries. After solution treatment β phases were soluted in α-Mg phase matrix . Aging treatment caused β phases precipitation. The corrosion resistance of magnesium alloy was determined in 3.5 % NaCl by immersion tests and polarization curves. The results shows that the corrosion resistance of magnesium alloy after solution treat is the best than that of the others heat treatment in polarization curve tests and the samples with aging for 16h and 24h presents higher corrosion resistance than those with solution and aging for 8h in immersion tests. Solution treatment plays a main role for corrosion resistance of Mg-7.3Al magnesium alloy in short term corrosion, but more continuous β phases in Mg-7.3Al alloy after ageing act as a barrier and play a main role in long term corrosion.