In order to recycle iron resources in nickel slag via modified molten oxidation used CaO as modifier under air atmosphere, the phase composition and microstructure of original and modified nickel slag, the distribution of elements was investigated, and the precipitation quantity and grain size of magnetite at various basicities were characterized, and the effect of basicity on the precipitation and growth in nickel slag were studied. The results showed that the iron element exists as fayalite and hortonolite. Fe, Ni, Co, Cu are simultaneously concentrated in magnetite phase during molten oxidation process In addition, the precipitation and growth of magnetite phase are affected significantly by slag basicity. When the basicity is about 0.6, iron is precipitated as magnetite with uniform distribution, and its precipitation quantity reaches a maximum value of 36.4%, the average size of larger than 50 ?m and distribution uniform.