Ultrasonic cavitation peening is an environmentally friendly technology to improve surface properties. In the traditional ultrasonic cavitation peening process, specimens have to be immersed in a liquid and temperature control is required, which limits the wide usage of this technology due to the geometry and complicated setup. In order to improve this process, water is slowly jetted (75 mL/min) into the gap between the sonotrode tip and specimen surface. The water jet makes the gap full of water. Thus, cavitation bubbles can be generated in the gap as the traditional ultrasonic cavitation peening process. In this case, the water container and temperature control are no longer necessary. The goal of this contribution is to evaluate the treatment effectiveness of this novel approach by the impact loads, the volume loss, the surface roughness, the microhardness and the microstructure of the specimen surface. The results indicate that a higher input power is beneficial and there would be an optimal gap width for this novel ultrasonic cavitation peening process.