Tool wear in micro ultrasonic machining (USM) affects the accuracy of a machined feature. Wear characteristics of a rotational cylindrical tool including shape alteration and longitudinal wear rate are investigated in this study. Three representative tool materials (AISI 316L stainless steel, tungsten, and cemented carbide) and slurry containing polycrystalline diamond abrasives are used. Two common tool shape alteration phenomena (flanging and rounding) have been identified. Microscopic observation of the tool tips provides an insight into the formation of flange and rounded edges. It is found that the low machining speed usually leads to flanging for AISI 316 and tungsten tools. The longitudinal wear rate of a flatended tool is measured under different machining conditions. Within certain ranges of parameters, the influence of workpiece material, tool material, tool diameter, static load, vibration amplitude, abrasive particle size, and tool rotational speed on longitudinal wear rate Is studied.