In the present study it was attempted to improve the performance of ultrasonic-assisted grinding using polycrystalline diamond (PCD) micropin tools processed by electrical discharge machining. The micropins acted as grinding tools because the convex parts of discharge craters can serve as cutting edges of abrasive grains in grinding wheels. The drilling conditions to prevent chippings and cracks on the hole edge were experimentally determined and then employed for the drilling of microholes less than 10 μm in diameter. As a result, microholes as small as 5.5 μm in diameter were successfully drilled in hard and brittle materials such as crown glass and monocrystalline silicon. The high wear resistance of PCD improved the tool life, making it possible to mill trenches and to mass-produce holes using a small number of tools. Moreover, the tool life was further improved by processing the tool peripheral surface at a low electrical discharge energy.