Abstract: An activity model of vanadium slag was established according to the coexistence theory of slag structure. The influences of vanadium slag compositions and temperature on the activities and activity coefficients of FeO and V₂O₃ were analyzed based on the model. The effects of vanadium slag compositions and temperature on the distribution behavior of V between slag and semi-steel at the end point of vanadium extraction by converter were studied by experiments and calculation. It is shown that the activity and activity coefficient of FeO, which are at magnitudes of 10-1 and 100 respectively, increase with the increase of FeO and MnO contents but decrease with increasing V₂O₃, SiO₂ and TiO₂ contents in the slag; the activity and activity coefficient of V₂O₃, which are at magnitudes of 10-2 and 10-1 respectively, show opposite trends compared with those of FeO under the same conditions. The mass fraction of V usually ranges from 0.02% to 0.06% in semi-steel, increases with the increasing of temperature and V₂O₃, SiO₂ and TiO₂ contents but decreases with increasing FeO content in the slag. The distribution ratio of V between slag and metal phase is in the range of 100 to 500, decreases with increasing temperature and SiO₂ and TiO₂ contents but increases while the content of FeO in the slag increases. There is a critical V₂O₃ content that makes the distribution ratio of V between slag and metal phase reach maximum, the theoretical value is 23.77% and the experimental one is from 15% to 20%.