The ultraviolet irradiation-assisted ultra precision polishing was performed on single crystal diamond substrates. This polishing method has been newly developed in our laboratory. The change of polishing performances was investigated by the presence of the UV irradiation. The polished surfaces were evaluated by the observation with WYKO. The experimental results are as follows; Surface roughness of diamond substrates polished under UV irradiation has become clearly smoother than that without UV irradiation. The surface roughness by this polishing method was reached to be 0.19 nm Ra on (100) surface of single crystal diamond. The equivalent surface was obtained on (110) surface by the UV-polishing.