Based on the calculation of energy density for several kinds of non-traditional machining process of engineering ceramics, the material removal mechanism for ceramics was studied. The studies revealed that the key reason for these technologies to machine ceramics effectively is high-energy-density. For Laser machining, EDM and plasma arc cutting, the material was removed by the high temperature ablation while the high density energy is applied to material surface. For high pressure abrasive water jet machining, the material was removed by erosion using the abrasive particles with high density energy. The ceramics was machined by micro-detonation arc under the synergy of ablation and erosion.