Non-equilibrium solidification of AZ91 magnesium alloy containing RE elements was performed by a copper mould casting method, and the effects of category of RE (RE=Ce, Er) and its content (0.25, 0.5, 0.75, 1 wt%) on the primary grain size, solute concentration and phase structures were researched. Results show that less solute content exists in α-Mg grain for Ce addition, accompanied by the serious solute enrichment ahead of solid/liquid interface and the formation of Al11Ce3 phase with high melting point. Consequently, the refinement effect of Ce is more significant than that of Er. With increasing the amount of Ce, both the primary grain size and the ratio of β-Mg17Al12 phase decrease continuously, with the dispersive distribution of eutectic phase in the matrix. In contrast, grain coarsening appears when the Ce content is larger than 0.75 wt%. On this basis, the refinement mechanism was explained according to nucleation and growth processes, which was further verified by the differential thermal analysis experiment.