The hypothesis of grain boundaries affecting anodic dissolution as well as the concept of their interaction with the electrolyte flow on the workpiece surface subjected to ECM is presented. Scanning electron micrographs provide supporting evidence. It is shown that grain boundaries are preferentially dissolved since they are sites of higher free-energy atoms and consequently have higher rate of metal ions leaving anode surface. Due to greater fluid agitation in boundary depressions they constitute also sites of higher mass transfer, increasing the diffusion rate of dissolved ions into the mainstream of the electrolyte flow. Thus, the more grain boundaries on the surface the better the conditions for dissolution. And since the analysis shows that smaller grains provide more boundaries per unit area, the smaller the grain size the higher the metal removal rate. Tests conducted in an experimental cell designed to provide defined hydrodynamic conditions are described and discussed. Results obtained for Hastelloy-X samples of three different grain sizes confirm the hypothesis showing that for workpiece structure with smaller grain size, higher metal removal rate and current density are obtained.