Abstract: Laser full-penetration welding of 304 stainless steel was carried out by YLS-6000 laser welding system.The effects of laser power and defocusing amount on formation of weld,microstructure and microhardness of T-joint were studied.Results showed that with the increase of laser power,weld penetration,weld width and lap weld width increased.The weld width reached the maximum when the defocusing amount was -2 mm.Also taking penetration-to-width ratio into consideration,formation of weld was better when the defocusing amount was 0 mm.The microstructure of weld zone was based on austenitic columnar crystal,and the network δ ferrite distributed in the gap.The hardness of weld zone was slightly lower than that of base metal,and with the increase of laser power,the microstructure of weld zone got coarsened and the hardness decreased.Highlights:Laser penetration welding was carried out on stainless steel T-joint,effect of laser power and defocusing amount on formation of weld was systematically analyzed.Considering weld width and penetration of lap joint,formation of weld was best when defocusing amount was 0 mm.