Abstract: The evolution of MX (M＝Ta; X＝C, N) phase in modeling steel during tempering at 750 ℃ was studied. The results indicate that TaC can be formed in reduced activation steels. TaC particles in two different scales were identified. The larger undissolved TaC particles exert strong pinning force on migrating of martensitic packet and prior austenite grain boundaries after austenitizing. The small TaC particles formed during tempering have an average diameter from 15 to 45 nm with the increase of tempeing time. The re-dissolution of the precipitated TaC particles during tempering can lead to the precipitating of new particles without causing the coarsening of the pre-existing particles, as a result, the coarsening rate of MX particles slows down. TaC particles grow along the particular atoms plane (200)_TaC, (111)_TaC during precipitation and coarsening.