The precipitation sequence of Al-Mg-Si alloys has been studied by high-resolution transmission electron microscopy (HRTEM). G.P. zone is a fine-plate having dimensions of a mono-layer in thickness, 2.5nm in width, and less than 30nm in length. Its elongated direction is parallel to the <100>(m) direction. Several types of metastable phases were observed in the excess Si alloy, and they were classified with the beta'-phase in the balanced alloy. Especially, the beta"-phase was the typical phase in the excess Si alloy aged at low temperature or at the early stage during aging.