In high-speed milling of aluminum alloy, flood coolant application has conventionally used to avoid adhesion of work material onto a tool surface. However great effort to reduce use of lubricant has being made to tackle growing environmental problems. Although MQL is one of the methods to reduce use of lubricant greatly, it can not successfully replace conventional flood emulsion because of less cooling ability. Oil-on-water droplet cutting fluid can cover the shortcoming of the MQL because it has large cooling ability. Furthermore, the OoW can reduce cutting force in comparison with that in the both cases of flood coolant and MQL applications. In this study, effect of the cooling ability of the OoW on reduction in cutting force is investigated in aluminum alloy milling. It is suggested that lubricant can effectively serve good performance and then reduce the cutting force only if tool surface can be sufficiently chilled.