Active flux can modify the fusion zone geometry dramatically in GTA welding (A-TIG). In the present study, in order to investigate the effect of the active flux on the Marangoni convection in the welding pool, bead-on-plate specimens are made on SUS304 stainless steel pre-placed with single active flux, Cu₂O, NiO, Cr₂O₃, SiO₂ and TiO₂ by the GTA process. Weld pool cross-sections and the bead surface morphology are analyzed by optical microscopy after welding. The oxygen content in the weld metal is measured using a HORIBA EMGA-520 Oxygen/Nitrogen Analyzer. The results showed that the depth/width ratio of the weld pool was closely related to the oxygen content in the pool. The oxygen content in the weld metal increases with the quantity of fluxes, Cu₂O, NiO, Cr₂O₃, SiO₂ and TiO₂. However, for the TiO₂ oxide flux, the highest oxygen content in the weld metal is below 200 ppm. As the oxygen content in the weld metal is in a certain range of 70–300 ppm, the depth/width ratio increases to 1.5 to 2.0 times. Too low or too high oxygen content in the pool does not increase the depth/width ratio. The oxygen from the decomposition of the flux in the welding pool alters the surface tension gradients on the weld pool surface, and hence, changes the Marangoni convection direction and the weld pool penetration depth.