Abstract: A weld pool dynamics approach has been used to investigate the weld peneration of a travelling arc in the GTAW process without filler.Because the weld ize and penetration are well related with the behavior of weld pool oscillation xcited by an arc force impulse,the size of weld penetration can be assessed by the weld pool oscillation frequency sensed directly through the arc voltage fluctuation.The weld pool oscillation can be excited by a sharp current pulse superimposed on the normal welding current when the weld penetration signal has to be detected.The obstacles encountered in this method for a faster travelling arc have been successfully overcome by using a external magnetic arc control.The satisfactory weld full penetration signals can be reliably detected even though the welding speed exceeds 200mm/min.The weld full penetration signals detected with this method offer a promising prospect for realizing the adaptive weld full penetration control without any external sensor in GTAW within normal or higher welding speed.