Abstract: Very high cycle bending fatigue test was carried out on 5083 aluminum alloy sheet with thickness of 2 mm and 5 mm at ultrasonic vibratory load, and the effect of thickness of aluminum alloy sheet on very high cycle bending fatigue properties of the sheet was studied. Results show that the S-N curves of the sheet exhibited continuous decline characteristics, the specimens fractured in the life cycle of 109. There was no traditional fatigue limit in the testing as conventional aluminum alloys used to be. Bending fatigue strength of the sheet with thickness of 2 mm was higher than that of the sheet with thickness of 5 mm, this was the result of size effect. In high cycle stage, the effect of size effect on fatigue property was the largest, and the effect gradually weakened in very high cycle stage. It was found that all fatigue cracks emanated from the specimen surface, and the crack was multiple sources. 5083 aluminum alloy sheet with different thicknesses had the same cleavage fracture characteristics at the same stress amplitude.