The effects of intensive melt shearing and Ca concentration on grain refinement of commercial purity Mg have been investigated by standard TP-1 casting tests, X-ray diffraction combined with the observation of optical microscope and scanning electron microscope. It was found that, without intensive melt shearing, Ca addition greatly improved grain structure of Mg from columnar grain to equiaxed grain. With Ca concentration increasing from 0.01 to 1 wt. %, the average grain size of the alloy gradually decreased. When intensive melt shearing was applied, the grain structures of the alloy remained equiaxed grain with Ca concentration variation. The average grain size first decreased and then kept invariable with Ca concentration increasing to 1 wt. %. The grain refining mechanism of Ca concentration variable and intensive melt shearing was discussed.