Abstract: The surface of ductile cast iron was treated by ultrasonic machining. The three-dimensional morphology, surface roughness and microstructure of the surface of non-machining area and machining area were studied by using white light confocal microscope, scanning electron microscope, optical microscope and Vickers hardness tester. The results show that the ultrasonic machining had little effect on the size of the sample, and the continuous polishing was achieved. The transition zone between non-machining area and machining area had a butterfly shaped protrusion with height of about 30 μm. The surface roughness of the sample was reduced by ultrasonic machining significantly. The arithmetical mean deviation of the profile changed from 1.465 μm before machining to 0.948 μm, being reduced by 35.29%. The grains in the surface layer of the sample were refined obviously after ultrasonic machining; the surface hardness of the sample increased. The effect depth of ultrasonic machining on the hardness of the sample was about 0.6 mm.