Aluminum is inferior in mechanical properties, such as hardness and wear resistance, compared with steel material, so various technical developments with which they are compensated is performed. In this research, the surface modification which used electrical discharge machining as one of the technology was observed. Processing liquid had high heat stability compared with the kerosene usually used by electrical discharge machining, and used silicone oil excellent in electric insulation. Stabilization of electric discharge and the increase in the thickness of EDC layer are expected by using this processing liquid. EDC layer speed was able to be raised to 378 M m/3min by optimization of pulse width, a current value, a duty factor, and a electrode gap.