In order to reveal the electrical discharge machining characteristics of electrical conductive alumina based ceramics, a series of EDM tests are carried out. The following main results are obtained. (1) The surface roughness of ceramics machined with negative polarity is larger than that with positive polarity. (2) When machining ceramics with small current, compared with positive polarity, the material removal rate of negative polarity is larger and the electrode wear ratio is smaller. But when machining with large current, the relation is reversed. (3) When machining with short pulse and negative polarity, the recast layer is thinner and almost no crack is observed because of the impact force caused between the workpiece and electrode is larger than positive polarity. (4) Compared with normal EDM, material removal rate machined with orbital motion of an electrode is higher, but the surface roughness and the electrode wear ratio are worse. But when the orbital distance of an electrode increases, the surface roughness and the electrode wear ratio are improved remarkably.