Polycrystalline diamond (PCD) has been widely used for various cutting tools and die components making use of its hardness and wear resistance properties. The polishing method of a single crystal diamond substrate and SiC using ultraviolet irradiation was newly developed to obtain mirror-finished surfaces. Due to the long polishing time in this method, a better pre-machined surface is required to shorten the total processing time. In this work, the constant-pressure grinding was performed using a cup type metal-bonded diamond wheel and a constant pressure device. After the good constant-pressure grinding, the PCD was finished by the polishing under the ultraviolet irradiation, and the microroughness was reached to be 0.71 nmRa.