The as-cast and heat-treated microstructures and mechanical properties of the AZ91 magnesium alloys with and without minor Sc addition were investigated and compared in this paper. The results indicated that adding 0.15–0.45 wt% Sc to the as-cast AZ91 alloy not only could modify and refine the Mg17Al12phase but also suppress the formation of the Mg17Al12phase. At the same time, the grains of the Sc-containing as-cast AZ91 alloys were also effectively refined.As a result,the mechanical properties at room temperature(RT)for the Sc-containing as-cast AZ91 alloys were effectively improved. In addition, adding 0.15–0.45 wt%Sc to the AZ91 alloy promoted the formation of the continuous precipitates(CP)during the aging treatment in spite of that the formation of the discontinuous precipitates(DP)was simultaneously suppressed.Accordingly,the Sc-containing as-aged AZ91 alloys obtained the relatively higher mechanical properties at RT than the as-aged AZ91 alloy.