Abstract: The activity model of quarternary system CaO-FeO-SiO₂-V₂O₃ was established according to the coexistence theory of slag structure.The results show that the activity coefficient of V₂O₃ in slag is in good accordance with experimental data.On the other hand,activity coefficient of V₂O₃ depends on the basicity and oxidizability of slag.The yield of vanadium depends on the steel composition,slag composition and slag quantity.The slag quantity and oxidizability have remarkable effects on the yield during making vanadium steel.Among some additives such as ferrovanadium,V₂O₅ and vanadium slag,the best yield of vanadium comes from ferrovanadium,while the poorest one is vanadium slag,which can be only used for microalloy vanadium steels.