Various amounts of antimony and mischmetal (Ce-rich) were added into AZ91 cast magnesium alloy and the microstructural characteristics of the cast magnesium alloys were studied by optical microscopy, electronic microscopy and X- ray diffraction techniques. Both antimony and mischmetal had grain refinement effect to AZ91 alloy. When they were added together, a further refinement was observed. SEM-EDX and XRD analysis indicated that a plate-shaped phase, Al(11) RE(3), formed at the boundaries of dendritic alpha-Mg grain. A rod-like Mg(3)Sb(2) compound distributed both within alpha -Mg dendrites and along grain boundaries. A new spherical particle, ReSb, formed in the magnesium alloy when O.gwt%RE and 0.4wt%Sb were join added. The individual additions of antimony or mischmetal to AZ91 alloy did not result in a noticeable improvement to its room temperature tensile properties, but when 0.8wt%RE and 0.4wt%Sb were added together, the as-east tensile properties of the alloy were increased dramatically.