Electro-conductive ZrB2-based composite ceramics, containing SiC and B4C, were machined with an electro-discharge machining (EDM) process. The EDMed surfaces were covered with resolidified ZrB2 layers. Many open pores and surface cracks were observed on the surfaces. The strength degradation of the ceramics caused by machining was evaluated by three point bending tests of the partially EDMed bending specimens. The effects of pulsed current, pulse duration and duty factor on the strength and the roughness of EDMed surfaces are discussed. The strength of EDMed specimens was increased with decreases in pulse current, pulse duration and duty factor. The roughness of EDMed surfaces was decreased with decreases in pulse current, pulse duration and duty factor. The reliability of the ceramics EDMed with the appropriate conditions was as high as that of the ceramics ground with a 400 grit diamond wheel. It would be possible to use the carefully EDMed ZrB2-based composite ceramics as structural components without any additional finishing processes.