The reaction equilibrium between slag-hot metal containing Nb at temperature of hot metal pretreatment was investigated. Hot metal and slag containing Nb_2O_5 from Baotou was used. Meanwhile, a_([O] in molten iron was measured by means of the elctrolyte oxygen cell technique for calculating a_(Nb_2O_5) in slag. It was shown that when [Si]>0.02% and SiO_2/MnO<2,at the equilibrium state between slag-hot metal the selective removal of [Si] over [Nb] from Baotou hot metal is quite possible. The range of γ_(Nb_2O_5) values is 10°—10~(-2) at 1350℃. SiO_2/MnO of slag effects considerably on distribution ratio of Nb between slag-hot metal and on γ_(Nb_2O_5).