Abstract: Magnesium alloy (AZ31B) and steel (Q235) were friction stir butt welded. Optimal joint was obtained by adjusting the processing parameters. The microstructure of joint was observed by the optical microscope. The interfacial morphology along the thickness direction was analyzed by scanning electron microscope. The result shows that metallurgical joint of magnesium and steel is achieved. The interface between spoonshaped area of nugget and magnesium side is obvious. The grains of this part are coarse compared with that of the base material. The microstructure in the thermo-mechanically affected zone near the steel is non-uniform,composed of equiaxed grains and strip grains. The thermo-mechanically affected zone near magnesium side is inconspicuous. The grains of heat-affected zone on the magnesium side grow coarser than that on the steel side. The microhardness of local area on the steel side increases with the distance getting close to the nugget,of which the microhardness is non-uniform. In local area,hardness was very high that the maximum value is 324.7 MPa,and it is uniform on magnesium side.