Abstract: The primary Mg₂Si phase of hypereutectic Mg-3.2Si alloys, in which rare earth Er and Ce were added, was studied by OM, SEM, EDS and XRD. And the metamorphic mechanism was also discussed. When 0.6% (mass ratio, the same below)Er to Mg-3.2Si alloys added, the size of the primary Mg₂Si phase decreases from 150 μm to 40 μm, and its shape transformes from the bulky dendritic into the irregular polyhedron shape. Then continuely adding 1.0% Ce, the polyhedron or globular primary Mg₂Si phase of 5-10 μm can be obtained. When the addition of rare earth increases higher, the primary Mg₂Si phase grows into a coarse. Rare earth Er and Ce on the primary Mg₂Si phase reduce the critical nucleation work and the relative growth rate between the crystallographic orientation, more primary Mg₂Si crystal nucleus form, and its shape transformes into the irregular polyhedron shape. The rare earth Er and Ce form a continuous solid solution during solidification. When 0.6% Er and 1.0% Ce added, the tensile strength σ _b and elongation δ of Mg-3.2Si reach to maximum value of 127 MPa and 3.7%.