Ultrasonic machining (USM) is known for its ability of processing brittle and hard materials such as silicon, glass and quartz. Usually, material removal in conventional USM is in brittle mode. The machined surface is covered with sharp tips and edges. In micro USM, the size of machined feature is less than 1 mm. Different from the conventional USM, in micro USM, it was found that the machined surface is flat and smooth under certain machining conditions. This indicates that the ductile material removal mode exists in micro USM. Based on the experimental observation, the surface roughness, Rpk, is used to identify the existence of ductile material removal mode in micro USM. The impact force of a single abrasive particle is calculated based on the elastic theory and crack generation. In this paper, 3D micro cavities were machined in quartz by micro USM under different machining conditions. Machined bottom surfaces were measured. Experimental results indicate that the brittle and ductile material removal modes are achievable by controlling the static load of micro USM.