Abstract: Cavitation erosion of Q235 steel in tap water and in deionized water was carried out by using an ultrasonic cavitation equipment. Then, surface morphology and phase structures of cavitated surface were investigated by using scanning electron microscopy, laser scanning confocal microscopy, Raman spectroscopy and transmission electron microscopy. The results show that the cavitated surface of Q235 steel exhibited a rainbow circle structure, which was composed of nano-sheet δ-FeOOH with different thickness. It deduced that the iron oxide nano-sheets were produced by cavitation induced eruption and pilling up. Compared to those in deionized water, the erupted nano-sheets produced in tap water were larger in quantity and thickness. Ions in tap water had influence on promoting the formation of iron oxide nanostructures.