The effect of $O_2$and Al contents and the variation of welding parameters such as arc currents, welding speed on the weld penetration was investigated. Examination of weld penetration using GTAW was accomplished in the ferritic STS410L and austenitic STS304. Good penetration could be controlled by the variation of $O_2$ and Al contents in STS304. However, influences of $O_2$ and Al contents on the ferritic STS410L are far less than those on the austenitic STS304. Welding parameters should be considered first before controlling $O_2$ and Al contents for a good penetration in ferritic STS410L. In the simulation study under the stationary heat sources, the results of simulation and experiment have a similar tendency.