403Nb is a kind of martensitic heat-resistant stainless steel which was developed from the traditional AISI 403 stainless steel by adding certain amount of Nb and V and widely used to manufacture the steam turbine blade of the heat-engine unit. Compared with the conventional normalizing and tempering treatment, thermo-mechanical treatment including normalization at elevated temperature, followed by warm-rolling in metastable austenitic phase and air cooling has been investigated on 403Nb steel in present study. Microstructural observations show that thermo-mechanical treatment leads to a finer martensitic microstructure pinned with numerous nano-sized MX carbide particles. The direct result of microstructural change is that the yield stress of the thermo-mechanically treated 403Nb steel is significantly improved compared with the normalized and tempered. The yield stress increases from 800MPa to 1180MPa at 20 ℃ and from 246.5 MPa to 341.4 MPa at 700 ℃, and the increasing ratio is about 47.55% and 38.5%, respectively.