The service life of a diamond tool in cutting single-crystal silicon is normally very short because of severe tool wear. Therefore, it is important to use a proper coolant in order to restrain tool wear. In this paper, the performances of oil-based and water-based coolants were compared in silicon machining by investigating cutting forces and tool wear geometries. The water-based coolant was found to restrain flank wear more effectively than the oil-based one. The effective tool life using the water-based one was averagely three times longer than that using the oil-based one. The tool wear mechanism might be related to microplasma generated between silicon and diamond during cutting.