Material removal mechanisms of Al2O3-based conductive ceramics machined by electric discharge milling in deionized water are investigated. The main components of conductive ceramic samples are Al2O3 (80%), TiC (11%) and WC (9%). The material removal mechanisms by low electric discharge energy are mainly spalling and melting. The material removal mechanisms by high electric discharge energy are mainly melting, evaporation and spalling. With high discharge energy and insufficient cooling conditions, chemical reactions will happen, where TiC and WC are oxidized to dielectric TiO2 and WO3, which will slow down the electric discharge milling processand finally make it stopped.