The effect of Ar gas bubbling on the tensile elongation of gravity mold castings of AZ91D magnesium alloy was evaluated qualitatively. The beneficial effect of gas bubbling on the tensile elongation of castings was resulted from the removal of inclusions in melt. The removal efficiency of inclusion by bubble floatation is dependent on processing variables including flow rate of gas, gas blowing time and melt temperature. Considering only the interaction between bubble and inclusion, the removal efficiency of inclusion will increase with increase in flow rate of gas, gas blowing time and melt temperature. But dissolution of gas into melt and entrance of new non-metallic particles formed on the melt surface deteriorated the tensile elongation of castings.