A　3D　mathematical　model　is　developed　to　calculate　the　temperature　and　velocity　distributions　in　a　moving　gas　tungsten　arc　(GTA)　welding　pool　with　different　sulfur　concentrations.　It　has　been　shown　that,　the　weld　penetration　increases　sharply　with　increasing　sulfur　content.　When　sulfur　content　increases　beyond　80×l0-6,　the　increase　in　sulfur　content　does　not　have　an　appreciable　difference　on　the　welding　pool　size　and　shape,　and　the　depth/width　remains　constant.　Sulfur　changes　the　temperature　dependence　of　surface　tension　coefficient　from　a　negative　value　to　a　positive　value　and　causes　significant　changes　on　flow　patterns.　The　increase　in　soluble　sulfur　content　and　the　decrease　at　free　surface　temperature　can　extend　the　region　of　positive　surface　tension　coefficient.　As　sulfur　content　exceeds　125×l0-6,　the　sign　of　surface　tension　coefficient　is　positive.　Depending　upon　the　sulfur　concentrations,　three,　one　or　two　vortexes　that　have　different　positions,　strength　and　directions　may　be　found　in　the　welding　pool.　The　contrary　vortexes　can　efficiently　transfer　the　thermal　energy　from　the　arc,　creating　a　deep　welding　pool.　An　optimum　range　of　sulfur　content　is　20-150×l0-6.