Abstract: Based on the introduction of the preparation method of great resistance medium and its application in resistance spot welding of aluminum alloy,the shape,defect and microstructure of nugget of aluminum alloy LF21 both with medium and without medium were studied by metallographic analysis and SEM.The results show that all of the nuggets without medium appear blowholes and lack of fusion,while those with medium appear well.There are only few small blowholes,but no lack of fusion which does more damage to mechanical property of the nugget in the latter situation.Furthermore,the requirement on the surface of aluminum alloy weldment is reduced,which avoids the treatment of weldment surface.The quality of aluminum alloy nugget can be guaranteed easily.The welding current can be reduced,which produces lower electrode temperature and enhances the electrode life.