The solubility of nitrogen in Fe-C-B-N system was measured at 1 758 K,and the computational model on activity (action concentration) of nitrogen and boron was established based on phase diagram and the coexistence theory about metal melt structure model. Comparing the computed results with the experimental results,satisfactory conclusion can be obtained. The result shows that BN and B4C can exist in Fe-C-B-N molten metal at high temperature,which consequently restrains the nitrogen removal from the melt. However,B4C content is extremely low. Before graphite is precipitated,the influence of carbon on activity of nitrogen in melt is higher in ternary system than in binary system; however,this effect is contrary to that after graphite is precipitated. The solubility of nitrogen in Fe-CBN system was measured at 1 758 K, and the computational model on activity (action concentration) of nitrogen and boron was established based on phase diagram and the coexistence theory about metal melt structure model. Comparing the calculated results The result shows that BN and B4C can exist in Fe-CBN molten metal at high temperature, which makes restrains the nitrogen removal from the melt. However, B4C content is extremely low. Before graphite is precipitated, the influence of carbon on activity of nitrogen in melt is higher in ternary system than in binary system; however, this effect is contrary to that after graphite is precipitated.