The electricity rust preventive machining method is proposed to effective prevent the workpiece from rusting in the cylindrical grinding only with water. The workpiece as a cathode is preserved from rusting by supplying the electric current through the water. This method leads to the ecology machining because the water without extreme pressure agents, mineral oils and the like is used as the machining fluid. Therefore, this method saves much work for a treatment of used machining fluid. Main conclusions obtained are as follows: (1) The effective prevention from rusting for 1 hour can be achieved by the electricity rust preventive machining method in cylindrical grinding. (2) The effective rust prevention conditions of electric supplying time and current are experimentally decided in cylindrical grinding and a very few electric power is necessary for rust prevention. (3) The supply current for the rust prevention increases with increasing the voltage, the nozzle-workpiece clearance or the exposed surface area. And the turbulent flow of water interferes with the supply of the current which increased with increasing the water flow rate. © 2005, The Japan Society for Precision Engineering. All rights reserved.