Effect of minor Sc, Zr and Ti co-addition and Mg content on the microstructure and Brinell hardness of Al-Mg alloys was investigated by means of Brinell hardness tester, optical microscope, SEM and TEM. Results show that the grain size of the as-cast alloys was refined by the addition of Sc and Zr, and it can be further refined by the addition of Ti. With the same Sc, Zr and Ti content, the increase of Mg content is beneficial to the refinement of microstructure, which is due to the solid solution strengthening of Mg in α-Al. The refined microstructures of the as-cast alloys were favorable for the improvement of the Brinell hardness. After homogenization annealing treatment of Al-10Mg-Sc-Zr-Ti, the Brinell hardness of alloys increased by 10% owing to second precipitations of Al3(Sc1-xZrx), Al3(Sc1-xTix) and Al3(Sc1-x-yZrxTiy), increased dispersity and more homogeneous distribution