Abstract: 309 stainless steel was tungsten inert gas welded with ZrO₂ active flux (with mass fractions of 0,0.25%,0.50%,1.00%, respectively) and the effects of ZrO₂ on the welding arc characteristc, fusion depth, and on the microstructure and property of weld were studied. The results show that when welding with ZrO₂, the arc illustrated pulse characteristics. The arc voltage decreased first and then increased with increasing mass fraction of ZrO₂. The fusion depth after welding with ZrO₂ was more than 2 times larger than that after welding without ZrO₂. The weld obtained by welding with 0.50wt%ZrO₂ possessed a microstructure of austenite and few δ ferrite, and the austenite grains in some regions were equiaxed. The addition of a small amount of ZrO₂ decreased hardness and yield strength in bending of the weld zone. With increasing mass fraction of ZrO₂, the hardness of the weld zone decreased, the yield strength in bending decreased first and then increased, the thermal conductivity decreased and the specific heat capacity increased.