Abstract: To determine the nature of the porosity, the gas compositions in porosity were tested by suspension meltinggas chromatography apparatus. The photograph of internal porosity was also observed by SEM. The analysis result shows that the gas compositions of porosity are the same mixed ones of CO and H₂ in both basic and acid electrodes. CO may be considered to play an important role in the formation of porosity. Two kinds of shapes are shown in the interior porosity. The one is worm-like porosity which situated in the middle and upper weld metal, and the other is conchoidal porosity which can be found at the root pass. The analysis result also shows that, whether the formation of porosity or nat, can be predicted by the diffusible hydrogen content of weld metal.