Abstract: An experimental of weld penetration of TC4 titanium alloys was carried out by using ultrahigh frequency pulsetungsten inert gas welding process.The effect of pulsed current frequency on weld appearance was investigated.The experimental results show that when pulse frequency f ≤ 65kHz,the variation of welds depth and width with the increment of pulse frequency are basically the same trend,and compared with conventional GTAW process,the weld width decreased by more than 12%.In the range of 20~40 kHz,the weld depth-to-width ratio increases obviously,while it tends to be stable with further increase of pulse frequency.The weld width decreases approximately by 35% as pulse frequency is higher than 70 kHz.Meanwhile, the weld depth increases slightly and the depth to width ratio can be enhanced strikingly with ultra-high frequency.Based on the data from experiment of arc profile,the most dominant mechanism for the variation of weld appearance can be considered as arc constriction caused by pulse frequency.