Abstract: Activity interaction coefficients between elements Ti and Mo in iron-based systems were estimated on the basis of the thermodynamical theory. The solubility of Ti, C and N in austenite of Ti-Mo micro-alloy steel from 800 to 1300℃ was theoretically calculated at different Ti and Mo contents. Mo can decrease the activity of Ti in iron-based systems and restrain the precipitation of Ti, but its influence is little. The influence degree decreases with the rise of temperature. When the Ti content is constant, the influence degree heightens with the increase of Mo content in the steel. But when the Mo content is constant, the influence strengthens firstly and then weakens with the increase of Ti content in the steel. Finally, the calculated results were compared with experimental data. It is indicated that when the atomic ratio of Ti/C is about 0.5, the mechanical properties of the steel are the best and the Mo content has the greatest impact on the precipitation of Ti(C,N).