The present study investigates the effects of carbon content, microstructure and working pressure on the metal removal rate and current efficiency in the electrochemical machining of carbon steels. Four different carbon contents and various heat-treatment procedures (quenching, tempering and annealing) were performed. Some important experimental results are shown, as follows: (1) The removal rate and current efficiency increase with carbon content. (2) The quenched microstructures and tempered microstructures have a greater removal rate and current efficiency than those of the annealed microstructures. (3) The workpiece machined at the low working pressure of 3-4 kg/cm2 has the greatest removal rate and current efficiency. (4) The machined surface roughness of all kinds of microstructure decreases with increase in the carbon content. (5) The roughness of the machined surface for annealed microstructures is greater than those of the quenched and tempered steels.