Ultrasonic vibration can lead to significant load reduction in metal forming, and this concept has been widely applied in microforming. Recently, we discovered that low-frequency mechanical vibration (less than 100 Hz) with micro-amplitudes also features the same effects. In this study, low-frequency vibration-assisted tensile deformation experiments were conducted on commercially low-carbon steel. Effects of vibration softening and residual softening were obtained during experiments. Both these softening effects became prominent at high vibration amplitudes. Detailed microstructural analyses reveal that a low-frequency vibration treatment altered the interior characteristics of the metal. Electron backscatter diffraction results showed low-angle grain boundaries, and the interior misorientation angle increased greatly with the application of a low-frequency vibration. Changes in the microstructure became more pronounced with the rise of vibration amplitudes. Instantaneous stress reduction results from the additional energy applied in the form of vibration, which lowers the barrier energy for the dislocation motion. The residual softening effect can be interpreted via a dislocation density decrease as a result of vibration markedly improving the opportunity for dislocation annihilation or stacking.