The precipitation of three new types of metastable phases, i.e., TYPE-A, TYPE-B and TYPE-C, with different crystal structures from the beta＇ phase is proposed from our research on the change in crystal structures and formation sequence of metastable phases during the aging of the Al-1.0mass% Mg2Si-0.4mass% Si alloy by a combination of analytical high resolution electron microscopy and energy dispersive X-ray spectroscopy. The sequence of their formation is explained as follows. First, precipitation of the beta＇ phase and TYPE-B precipitate, then beta＇ dissolution into the matrix and degradation of the TYPE-B precipitate. Finally, predominant precipitation of the metastable TYPE-A precipitate. The TYPE-C precipitate appeared heterogeneously in the over-aged condition.