Abstract Authors investigated the effect of ultrasonic vibration on the solidification microstructure and mechanical properties of the weld metal of ferritic stainless steel by introducing directly ultrasonic vibration into the weld molten pool using ultrasonically vibrating filler metal. The main results obtained in this study are as follow. The ultrasonically vibrating filler metal could successfully transmit ultrasonic vibration to the weld molten pool. Ultrasonic vibration encouraged equiaxed grains to form in the central region of the weld metal. The more equiaxed grains formed due to ultrasonic vibration, the higher the welding speed was. The tensile strength of the weld with ultrasonic vibration was larger than that without vibration. The tensile fracture elongation of the weld with ultrasonic vibration remarkably increased compared to that without vibration. When the distance between an electrode tip and the filler metal was too short, the weld bead was less stably formed.