A　3D　mathematical　model　was　developed　to　simulate　the　weld　pool　development　in　a　moving　A-TIG　weld　pool　with　different　oxygen　and　sulfur　concentrations.　It　is　shown　that　the　surface　active　elements　-　oxygen　and　sulfur,　which　change　the　temperature　coefficient　of　surface　tension　from　a　negative　value　to　a　positive　one,　can　cause　significant　changes　in　fluid　flow　patterns　and　the　weld　penetration.　When　surface　active　element　content　increases,　the　weld　penetration　and　depth/width　ratio　increase　sharply　and　then　remain　nearly　a　constant.　Positive　temperature　coefficient　of　surface　tension　dominates　the　fluid　flow　and　the　weld　pool　is　narrow　and　deep.　The　further　increasing　surface　active　element　content　leads　to　an　inappreciable　difference　in　the　weld　pool　size　and　shape　when　the　oxygen　content　increases　beyond　280　ppm　and　sulfur　content　beyond　125　ppm.　Positive　and　negative　temperature　coefficients　of　surface　tension　co-exist　in　the　weld　pool　when　surface　active　element　content　is　less　than　the　critical　value.　The　fluid　flows　in　the　weld　pool　change　apparently　with　different　surface　active　element.　Depending　upon　the　oxygen　and　sulfur　concentrations,　three,　one　or　two　vortexes　that　have　different　positions,　strength　and　directions　may　be　found　in　the　weld　pool.　The　vortexes　with　opposite　direction　caused　by　positive　temperature　coefficient　of　surface　tension　can　efficiently　transfer　the　thermal　energy　from　the　arc,　creating　a　deep　weld　pool.