Insulating ceramics can be machined by electrical discharge machining (EDM) with an assisting electrode method. During the machining process, a carbonization phenomenon occurs on the workpiece surface continuously due to cracked carbon from working oil. And the carbonized surface has electrical conductivity. However, because of discharge concentration and discharges with long pulse duration, the machining characteristics such as removal rate and surface roughness are inferior to those of he electro conductive materials. This paper describes the machining rate of insulating Si_3N_4 ceramics by this method in which several kinds of powder are suspended in working oil. With help of powder suspended oil. the removal rate was improved due to the decrease of discharge concentration. The removal rate in drilling Si_3N_4 by EDM leads almost the same by grinding process.