Welded parts fabricated from high-strength steel (HSS) require an almost infinite lifetime, i.e., a gigacycle (10 9 ). Therefore, it is necessary to test its high-cycle fatigue behavior. In this paper, an accelerated fatigue test method using ultrasonic resonance is proposed. This method reduces the experimental time required in comparison with a conventional fatigue test setup. The operating principle of the accelerated ultrasonic fatigue test involved the use of a 20-kHz resonant frequency. Therefore, it was necessary to design a specimen specifically for the test setup. In the study, ultrasonic fatigue testing equipment was used to test butt-welded 590-and 780-MPa ferrite-bainite steel plates. In order to design the specimen, a dynamic Young's modulus was measured using piezoelectric element, a laser Doppler vibrometer, and a digital signal analyzer. The S-N curves of fatigue behavior of the original and butt-welded specimens were compared. The fatigue test results showed that the infinite (i.e., gigacycle) fatigue strengths of the welded specimens were approximately 8% less than those of the original specimen.