The method of electrical discharge machining (EDM), one of the processing methods based on non-traditional manufacturing procedures, is gaining increased popularity, since it does not require cutting tools and allows machining involving hard, brittle, thin and complex geometry. By using different EDM parameters (current, pulse on-time, pulse off-time, arc voltage), the R. (mu m) roughness value as a result of application of a number of copper electrode-hardened powder metals (cold work tool steel) to a work piece has been investigated, in this study. At the same time, roughness values obtained from the experiments that have been modeled by using the genetic expression programming (GEP) method and a mathematical relationship has been suggested between the GEP model and surface roughness and parameters affecting it. Moreover, EDM has been used by applying copper, copper-tungsten (W-Cu) and graphite electrodes to the same material with experimental parameters designed in accordance with the Taguchi method. Results obtained from this study have been compared among each other and similar studies in the literature. (c) 2007 Published by Elsevier B.V.