The work aims to study ultrasonic cavitation erosion behavior and mechanism of low-carbon and 304 stainless steels in distilled water, and evaluate cavitation resistance for low-carbon and 304 stainless steels to provide a basis for selecting cavitation resistant materials. An ultrasonic cavitation erosion test rig meeting ASTM international standards was used to conduct ultrasonic cavitation experiments of low-carbon and 304 stainless steels in distilled water for different time. The ultrasonic cavitation behaviors of the two materials were described and analyzed contrastively through cumulative mass loss (weight loss), cumulative mass loss rate (weight loss rate), surface morphology and residual stress. Low carbon steel sample entered acceleration stage after 15 minutes of cavitation, had shorter stabilization stage around 90 minutes of cavitation, and then quickly reached attenuation stage as cavitation time increased. The cumulative weight loss rate of the 304 stainless steel sample changed slowly within 30 minutes of cavitation, and then increased sharply with the increase of cavitation time. After 120 minutes of cavitation, the 304 stainless steel sample entered the attenuation stage. Slip effect dominated the deformation of the two steels during the cavitation erosion process. As the cavitation erosion developed, grains of the low-carbon steel underwent consecutively steps of grain orientation, grain refinement, grain boundary cracking, grain fragmentation and grain flaking. Under the same experimental conditions, corresponding variations of 304 stainless steel were lagging relative to those of the low-carbon steel. The residual stress of 304 stainless steel was relatively high. The extension of cavitation erosion cracks into 304 stainless steel is hindered by the austenite phase, thus significantly inhibiting the growth of cavitation erosion and improving the resistance to cavitation erosion of 304 stainless steel.