This work proposed a novel method of determining the energy distribution and plasma diameter of EDM. The energy distribution and plasma diameter were determined by comparing the boundary of the melted material in the crater which was obtained by metallographic method and the isothermal curve of the thermal-physical model using finite element method (FEM). The results of this work indicated that the expansion of the plasma diameter must be taken into consideration in order to be more consistent with the actual EDM process. With this method, the energy distribution and plasma diameter in different dielectrics with different polarities were investigated. Comparison between the results of this work and the previously reports showed that the energy distributed into workpiece and plasma diameter can be determined by this new method. (C) 2014 Elsevier Ltd. All rights reserved.