Abstract: TC4 Ti alloy was welded by conventional tungsten iner-gas arc welding(TIGW) and arc-ultrasonic TIGW respectively. The microstructure of the joint was analyzed by means of optical-microscope, scanning electron microscope to study the relationship between the macro-properties of the joint and the microstructure. The results show that the joints were welded successfully by conventional TIGW and arc-ultrasonic TIGW. With the increment of ultrasonic frequency and activated voltage, the width of the joint became narrow step by step. The microstructure became more and more fine and was inclined to equiax crystal. Moreover, the dendrite depredation was not observed obviously. The properties of welded joint were improved compared with that of conventional TIGW.