The objective of this paper is to reveal the main cause of volume effects of ultrasonic vibrations on the plastic behavior of pure aluminum specimens. For this purpose, specimens with different grain sizes were made by ECAP. An experimental tensile test system was designed and made, in which the specimens could be excited by ultrasonic vibrations with a frequency of 20 kHz and amplitude of 5 μm. Five specimens with grain sizes of 109, 38, 15, 7 and 0.97 μm were prepared. Tensile tests of the specimens were performed at room temperature and at constant speed of 0.2 mm/min under static load and superimposed ultrasonic excitations. It was found that ultrasonic vibrations had a remarkable influence on the plastic behavior of pure aluminum and after applying ultrasonic vibrations, flow stress of the all specimens reduced. Reduction of flow stress was dependent on grain size. The specimens with the largest grain size of 109 μm showed a flow stress reduction of 66% while finest grain size of 0.97 μm, a reduction of 11.3% was observed. The result of the current study can help to understand the underlying mechanisms of ultrasonic softening.