In　order　to　assess　the　effect　of　stress　triaxiality　on　the　plasticity　and　ductility　of　a　mild　steel　and　the　associated　weld　metals,　the　tensile　tests　on　notched　round　bar　specimens　were　performed.　The　results　indicated　that　the　strains　to　initiate　failure　were　strongly　dependent　on　the　stress　state.　Under　the　same　triaxiality　the　failure　initiation　strain　in　the　weld　metal　of　E6015　low　hydrogen　type　electrode　was　the　largest,　and　that　of　E6013　lime-rutile　type　electrode　was　the　smallest　among　the　materials　tested.　The　material　constant　C　values　relating　to　the　void　growth　kinetics　were　given　for　the　steel　and　weld　metals.　Moreover,　it　was　determined　that　rapid　void　growth　started　to　take　place　at　different　strains　for　the　materials.