The metastable phases in an Al-Mg2Si alloy containing silicon in excess were observed using HRTEM. The observed phases were found to be new three types with different crystal structures from that of the β′ phase (HCP lattice, a=0.407nm, c=0.405nm). The first precipitate has a hexagonal lattice with a=0.405nm, c=0.67nm (the TYPE-A precipitate), and the second one has an orthorhombic lattice with a=0.684nm, b=0.793nm and c=0.405nm (the TYPE-B precipitate). The third one, which is the target in this work, has a hexagonal lattice with a=1.04±0.02nm and c=0.405nm.