Friction stir welding of AZ31B magnesium alloy with 4 mm in thickness and Q235 steel was conducted. The microstructure and distribution of elements in various interface position between magnesium and steel in thick direction were analyzed by metallographic analysis、SEM （scanning electron microscope）、EDS and EPMA. In addition

effects of process parameters on mechanical properties of the joint were investigated. The results show that magnesium and steel exhibit metallurgical bonding

and the interface between welding nugget and steel presents shape "C". The interface shape is changed from irregular to straight line along the sheet thickness direction from the welding nugget top to bottom. There exists a transition region in the side of steel

which contains both Fe and Mg. With the welding speed of 60 mm/min

the tensile strength is increased firstly and then decreased with the increase of rotary speed. The maximum tensile strength reaches to 176.5 MPa when the rotary speed is 600 r/min

which reaches to 67.9% of strength of the base metal AZ31B. The tensile strength in the middle region of the joint is bigger than that in the top and the bottom region

which reaches about 195.9MPa and 75.3% of strength of the base metal AZ31B.