The authors investigated the effect of ultrasonic cavitation on the surface of St3, 20, 45 steels. It was found that cavitation causes a change in the structure and properties of the surface layer of the samples. A refinement of the grain and an increase in the microhardness of the surface layer similar for the investigated materials were revealed. The dependences of dynamics of the increase in microhardness under the action of ultrasound were given, which show that there was a steady state, characterized by an indicator of relative microhardness of 1.3...1.5. The results of changes in the parameters of roughness and sub-roughness were given. Studies showed that cavitation erosion leads to an increase in altitude and step parameters of the surface microgeometry. The research results can be used for development and creation of ultrasonic technological processes carried out in liquid media.