When the metal is melted by an arc plasma heating, convective flow is generated in the molten metal zone by various drag forces. The state of arc plasma used in this process is dependent on the current, voltage, shielding gas, and the evaporated component of the base material also affects the arc plasma. In this study, we show that oxygen content of the anode material affects the constriction of the arc plasma and changes the flow of molten metal by experiment and numerical analysis. First, after the generation of the arc plasma on ferritic stainless steels, which had different oxygen content and all other components were the same, the size of the arc plasma was compared by captured image using high speed camera. This size can be deduced from the brightness of the image, and it can be seen that the sizes of the two plasmas are slightly different from each other. Numerical simulation was carried out to confirm that these differences could affect the flow of the molten metal and final weld bead shape. As a result, the flow of the molten metal was completely different even with this slight change of plasma shape. These differences in molten metal flow was also observed in an experiment using a high speed camera. This change in flow can be explained by the interpretation that the Lorentz force is strengthened by the constriction of the arc plasma and the magnitude of the Marangoni force is increased by the increasing of thermal gradient. In conclusion, it was clearly confirmed that the oxygen content in the anode material affects the size of the fusion zone during arc plasma heating. (C) 2018 Elsevier Ltd. All rights reserved.