The welded joints of 400 MPa ultra fine grained steel in manual arc welding were treated by mechanical surface hardening. Microstructure and mechanical properties of the treated joints were compared with those of the untreated joints,based on which, primary study on the process and principle of mechanical surface hardening was carried out. The results shows that: Grain size of HAZ increases greatly and mechanical properties of welded joint decrease obviously compared with those of base material, but grain size in the surface layer of HAZ can be refined ( the grain size is about 100 nm or so) and mechanical properties of welded joints can be improved greatly by mechanical surface hardening.