A simple method for polishing of curved CVD diamond film with silicon nitride ceramics in air is proposed. The polishing experiment was carried out using silicon nitride ceramics with different components, and the surface roughness, polishing temperature, and amount of wear of silicon nitride ceramics were examined. The results showed that silicon nitride ceramics are effective as abrasives when the silicon nitride particle size is large and both hardness and porosity are low. These ceramics increased polishing temperature and decreases the amount of wear on the ceramics themselves. The optimum silicon nitride ceramics are discussed. It is expected that silicon nitride ceramics will have a marked effect on the polishing of CVD diamond films.