In this study, a series of experiments were conducted in order to investigate the mechanisms of forming force reduction and martensitic transformation in SUS304 stainless steel during ultrasonic vibration assisted tensile tests. An independent analysis of the impact of the stress superposition effect in the ultrasound assisted tensile tests were analyzed by finite element simulation, and metallographic analysis on the specimens showed that the increase of the amplitude of the ultrasonic vibration has a great influence on the martensite transformation in the material during deformation, reducing the elongation of the specimens. The results also pointed out that the reduction of the forming force is caused by a combination of the effects of stress superposition and the energy absorption of dislocations over the hardening of the material due to the increase of dislocation density and induced martensitic transformation.