Various opinions still exist regarding the grain refinement of magnesium alloys resulting from Ce addition. We investigate effect of Ce addition on the ascast microstructures of AZ31 magnesium alloy. The research results indicate that adding from 0.5 wt% to 1.5 wt% Ce to AZ31 magnesium alloy does not effectively refine the alloy grains; instead, the opposite effect results, causing obvious grain coarsening. The coarsening trend is related to the amount of Ce. After adding 0.5 wt% Ce to AZ31 magnesium alloy, the average alloy grain size increases from 60 μm to 164 μm. Furthermore, increasing the amount of Ce added from 0.5 wt% to 1.5 wt% causes the average grain size to decrease gradually. The average grain size of AZ31 magnesium alloy with and additional 1.5 wt% Ce still exceeds that of the AZ31 magnesium alloy without additional Ce, however.