The microstructures and phase composition of as-cast AZ91 magnesium alloys with addition of 2.5wt% RE (RE=La, La and Nd or Nd) in total were investigated by SEM/EDS and XRD. The mechanical properties of the alloys were analyzed at room temperature. The results show that the β-Mg17Al12 amount of AZ91 alloy is decreased evidently with the 2.5wt% addition of RE. The Al-RE phases of Mg-9Al-1Zn-2.5La and Mg-9Al-1Zn-2.5Nd are acicular Al11La3 and particle-like Al2Nd, respectively. Both Al11La3 and Al2Nd are formed with co-addition of La and Nd, whose sizes are smaller than those of single addition. The relative content of the two Al-RE phases is proportional to the addition content of La and Nd. Part of La in Al11La3 or Nd in Al2Nd is substituted by Nd or La respectively when La and Nd are co-added into AZ91 alloy. Compared with single addition of La or Nd, co-addition can improve the mechanical properties of as-cast AZ91 alloy more significantly. In all magnesium alloys investigated, the Mg-9Al-1Zn-1.0La-1.5Nd alloy exhibits the best mechanical properties, whose UTS is 235 MPa and d is 10% at room temperature