Abstract: In this paper, several theoretical problems about co-reduced Fe-Mo powder are discussed. Thermodynamical results show that MoO₃ or FeO·MoO₃ can be reduced by C or CO easily, Through mathematical analysis of diffusion equation, it is pointed that the value of Dt/l² could be a criterion representing the homogenity of element distribution. If it is far below one,diffusion proceeds without homogeneity. When mixed Fe and Mo powders were sintered at 1120-1180℃, Mo could diffuse into iron matrix. In the situation of co-reduced process,the corresponding value of Dt/l² is far above one. Mc diffuses very homogeneously in iron matrix. Lattice constants of pure Fe and Fe-Mo powders were measured. It is 0.28664nm for Fe,and it is 0.28673nm for Fe-Mo. It is suggested that Mo should be solved in the lattice of Fe. The sites of Fe are partly replaced by slightly bigger atoms of Mo, therefore the lattice constant of Fe-Mo alloy powder is enlarged.