Abstract: The lap welding of AZ31B Mg alloy to DP600 galvanized steel were successfully achieved by friction stir spot welding (FSSW) and vacuum diffusion welding. Scanning electron microscopy, energy disperse spectroscopy and tensile tests were used to investigate the microstructures and mechanical properties of the welded joints at the best welding parameters. The results show that there is no keyhole found in the FSSW joint and the good weld appearance is obtained. The bonding mechanism of the joints are mechanical bonding and metallurgical bonding. Intermetallic compounds of Mg, Fe and O formed because of the diffusion at the interface of magnesium and steel during the metallurgical bonding process. In the mechanical bonding process, some of the steel insert into the magnesium alloy substrate like nails and there exists serrated combination morphology at the interface between Mg and steel, the others is mixed into the magnesium alloy under the stir effect of stir pin which greatly improved the mechanical properties of joint of magnesium alloy to steel. For both the kinds of bonding mechanism, about 75.9% of the mechanical properties is contributed by mechanical bonding and mechanical bonding is the main bonding mechanism in the FSSW process.