A　three-dimensional　mathematical　model　was　developed　to　simulate　the　flow　patterns　and　temperature　distributions　in　a　moving　A-TIG　weld　pool　of　304　stainless　steels　with　different　oxygen　content　using　PHOENICS　software.　It　is　shown　that　the　surface-active　element,　oxygen,　is　important,　because　it　affects　the　weld　shape　by　changing　the　flow　patterns　in　the　weld　pool.　The　weld　bead　penetration　and　the　depth/width　ratio　increase　first　sharply　and　then　remain　nearly　a　constant　with　increasing　oxygen　content.　Depending　upon　the　oxygen　contents,　three,　one,　or　two　vortexes　that　have　different　positions,　strength,　and　directions　may　be　found　in　the　weld　pool.　Oxygen　can　cause　significant　changes　in　the　weld　shape　by　varying　the　sign　of　the　surface　tension　coefficient.　The　situation　with　the　maximum　surface　tension　moves　from　the　edge　to　the　center　with　increasing　oxygen　content.　As　oxygen　content　exceeds　a　critical　value,　a　positive　surface　tension　coefficient　dominates　the　flow　patterns.　The　vortexes　with　opposite　directions　caused　by　positive　surface　tension　coefficient　can　efficiently　transfer　the　thermal　energy　from　the　arc,　creating　a　deep　weld　pool.　The　critical　oxygen　content　increases　with　the　increase　of　the　welding　current.