The age hardening behavior and the change of the microstructure in Mg-Al system alloys containing 10 to 14 % Al were investigated by optical microscope, scanning electron microscope and micro-Vickers hardness measurement. When Al content was lowred 10 mass%, β phase is almost solid solution in the α-Mg matrix by the solution treatment. On the other hand, Al content was overed 11 mass%, the β phase remained at grain boundary and cell-gaps in the α-Mg matrix. As the solution treatment, the hardness was slightly increased with the increase of the Al concentration. When aging temperature is constant, the time to reach the maximum hardness is almost the same if differ in aluminum concentration. Therefore the difference of cellular precipitation sequence is not important for Mg-Al system alloys with containing of high concentration Al.