Abstract: The microstructure and mechanical properties of Ti-6Al-4V titanium alloys joints was studied, which obtained by ultra high frequency pulse gas tungsten arc welding(UHFPGTAW) process on the condition with 20% duty cycle. The degree of grain refinement was characterized by the average grain size in fusion zone(FZ). The characteristic of UHFP-GTAW joints microstructure was investigated, and the effect of pulsed current frequency on grain refinement and mechanical properties was also analyzed under condition of low duty cycle. The results showd that in contrast with conventional GTAW(C-GTAW) process, the microstructure of the FZ remained mainly basketweave, and most of α' phase of the HAZ presented as shortacicular for UHFP-GTAW process. The grains were refined significantly and the average grain size in FZ was decreased by 32% at most. Elongation and reduction of area were increased by 140% and 275%, respectively. Morerover, the ductility of the joints was improved strikingly. In comparision with the 16.7% decline of average grain size in FZ on 50% duty cycle, grains were refined further, and elongation and reduction of area were respectively enhanced by 22% and 33% as the duty cycle was reduced to 20%.