Abstract: Welding penetration control is crucial for weld quality control in GTAW of thin plates. Sensing of weld depth and recognition of full penetration is the key challenge in welding penetration control. Conventional direct vision sensing of full penetration at the back side of weld beam can effectively extract the penetration information, but often cannot be used due to the limited space accessibility. Therefore, in this paper, welding penetration control based on arc voltage sensing was studied. By extracting the penetration features during arc voltage sensing as a feedback, the welding current waveform could be modulated to adjust the welding depth in real time and thus to ensure uniform weld full penetration without burning through. The bead-on-plate welding experiments were conducted using 4 mm thick hot rolled steel plates and 3 mm thick cold rolled steel plates. Control principle was verified by enforcing the bead-on-plate welding experiments under different welding currents. Sound welding seams was acquired in variational cooling condition and variational clearance bead-on-plate welding experiments. Test results show peak voltage time can be controlled according to the intelligence feedback of arc voltage.