Abstract: The effect of applied longitudinal magnetic field on Co-based plasma arc surfacing on low carbon steel was studied.It is found that only the magnetic intensity and weld criterion parameters must be matched properly,the optimal effect of grain refining can be gained.And the discussed emphasis is the influence of magnetic density and welding current on the regularity of hardness and wear resistance of the overlay welding metal.The result indicated that proper electromagnetic field can refine the structure of overlay weld metal but also improve the hardness and wear resistance of the overlay welded mental,which increase the comprehensive mechanical properties of overlay welded metal.