Manufacturing accounts for approximately 30% of the energy use of the U.S. every year. Machining, including milling, turning, and drilling, accounts for about 80% of the manufacturing energy use. End milling has wide applications in manufacturing of die and mold. However, energy efficiency in end milling process is low and needs to be improved. The low energy efficiency not only leads to high production cost but also causes negative environmental impacts such as intensive CO2 emission. Therefore, a thorough study on energy consumption during end milling is highly needed. This research starts with a literature review of energy consumption and other energy related issues in machining processes. A systematic energy measuring method using high resolution power analyzer in end milling of AISI H13 steel has been developed. A feasibility and sensitivity study were conducted to evaluate the capability of power analyzer in data acquisition. Then, power and energy variables in end milling were comprehensively characterized. Power and specific energy were measured and correlated to process parameters [axial depth of cut, radial depth of cut, cutting speed, feed per tooth, and material removal rate (MRR)] in end milling. The power and specific energy for the same MRR by different combinations of process parameters was also investigated. Finally, the influence of up milling and down milling on power and specific energy was studied.