Micro square holes are often used in the industries of aeronautics and astronautics, ordnance and instrument, and are difficult to machine. To improve machining precision of micro square holes of metal material, a series of orthogonal experiments of micro electrochemical milling machining for micro square holes (50 μm×50 μm) of nickelplated stainless steel were carried out based on elaborating work principle and setup of micro electrochemical milling machining technology. Then effects of process parameters (voltage, electrolyte, feeding speed and pulse width) on machining precision were discussed. The results show that machining precision of micro electrochemical milling machining for micro square holes of nickelplated stainless steel can be improved by using low voltage, low concentration of electrolyte, high feeding speed and narrow pulsewidth power supply within a reasonable range of these parameters. 