There have been many studies on the mechanism of electrical discharge machining. However, the general principle of the machining has not been found yet because of its complex machining phenomena. This study aims to find a simple but practical law of removal rate related with thermal properties of work piece. The removal rate is almost determined by the product of thermal conductivity and melting temperature. And the effect of carbon layer which covers copper electrode on the wear of the electrode is discussed through the experiments with several kinds of materials of work pieces. The materials of work pieces are classified into two groups according to the wear rate of copper electrode.