Abstract: Ultrasonic assisted TIG welding is a new technology with high efficiency.In previous study, direct current was used.However, pulse current was expected in actual welding production.In this study, ultrasonic assisted pulse TIG welding technology was applied to weld 1Cr18Ni9Ti austenitic stainless steel.Microstructure and mechanical properties of the joints were analyzed.The reasons for weld penetration increasing and microstructure refinement were discussed.Experimental results indicated that the welding penetration with ultrasonic vibration was double of that without it.The microstructure of the weld zone was refined and the arc shape was compressed in base current period.The ultimate tensile strength and elongation were higher than that without ultrasonic vibration.Weld penetration increasing is perhaps attributed to acoustic streaming as the main driving force.The refinement of microstructure may be caused by the corporate effects of acoustic cavitation and acoustic streaming.