For development of high strength magnesium alloys, a melt stirring method was employed to Mg-Al and Mg-RE (rare earth metal) alloys. As reinforcement particle, B2O3 powder was added into melts and stirred at 953K in 10 or 20 minutes with argon atmosphere. The grain sizes of both alloys were decreased by 3wt% B2O3 addition. These results show that B2O3 have grain refinement effects to magnesium alloys. Micro Vickers hardness of Mg-Al alloy was increased by 3wt% B2O3 addition. On the other hand, the hardness of Mg-RE alloy was decreased by B2O3. Though addition of B2O3 into Mg-Al and Mg-RE systems make grain size to fine, the hardness of Mg-RE alloys decreased. The tensile properties of Mg-RE alloys with B2O3 were extremely improved by extrusion process.