Literature study and thermodynamic/kinetic analysis have been carried out on the effect of carrier gas on hot metal desulfurization by magnesium injection. The literature study shows that the magnesium efficiency of the process could be deteriorated by using nitrogen as carrier gas, but the difference in magnesium efficiency for the process using argon and that using nitrogen was not clearly identified. Thermodynamic/kinetic analysis shows that when nitrogen is used as carrier gas for introducing magnesium into hot metal, the formation of magnesium nitride is possible in the regions close to the lance tip. The nitride formed at lance tip may cause lance clogging. Magnesium nitride is unstable in hot metal or in gas at high temperatures; and after leaving lance tip regions, magnesium nitride will undergo decomposition. Magnesium loss in process off gas will be increased by the decomposition of magnesium nitride that occurs too closely to bath surface or by any un-decomposed magnesium nitride at bath surface. The magnesium loss by nitrogenization and the clogging problem could be minimized by optimizing injection conditions.