Abstract: A correlative formula which shows the relationship of temperature,nitride capacity and optical basicity was obtained by regression analysis.Two kinds of reaction mechanisms of nitrogen entering into slags were provided.The isograms of nitride capacity in CaO-SiO₂-Al₂O₃-MgO (w_MgO=10%) melts were depicted by regression equations.The results show that the nitride distribution ratio L_N increases with decreasing%Al_s in steel when the nitride capacity is a constant and L_N increases remarkably with increasing nitride capacity when the%Al_s is a constant.The interface between slags and molten steel with high nitride capacity has strong ability to reduce nitride in molten steel.Slags play an important part to remove nitride in molten steel and prevent from nitrogen pick-up during the production of ultra low-nitride steel whose solute nitrogen content is blow 3×10^-5.