Grain refinement of cast magnesium alloys containing Al is attained by adding carbon materials. This technique is well established. However, it is important problem that C₂Cl₆ as grain refiner generates dioxin matters. Therefore, it is impossible to use grain refiners containing chloride. This study was performed to develop a new grain refiner containing C element instead of harmful C₂Cl₆. Carbon is an important element for grain refining of the commercial AZ91 magnesium alloy. Pure carbon (99.9%) powder with high-purity argon gas carrier was blown into the molten AZ91 magnesium alloy. Fine grains were obtained by addition of the carbon powder. Foreign substances as nucleants could be observed in the center of each grain. The result of EPMA analysis indicated that the foreign substance was consisted of Al, C and O.