Equilibrium between slag and molten iron at 1600℃ was investigated for the direct alloying of steel with Nb
2O
6-bearing slag. The activity of niobium oxide in the slag was measured. The α
Nb2O5 is in the range of 10
-2-10
-4. Theδ
Nb2O5 is ranged in 10
0-10
-2.Niobium partition decreases with the increase of[Si]% and CaO/SiO
2. When α
[0] and ΣFeO decrease, niobium partition decreases too. The δ
Nb2o5 and the ratio of γ
siO2 to γ
Nb2O5 decrease with the incre-ase of slag basicity. The suitable thermodynamic conditions in direct alloying of steel with Nb2O5-bearing slag have been found.