An optimization method of end mill geometry parameters is presented for minimizing cutting energy. The helical end mill geometry is established at first. Then, the helical flutes are decomposed a set of infinitesimal oblique cutting edges. At every oblique cutting element, the differential cutting energy, which consists of differential shear energy and differential friction energy, is calculated using oblique cutting theory. By integrating the differential cutting energy along each cutting edge in the end mill, the cutting energy can be predicted during end milling. The effects on cutting energy of end mill geometry parameters are analyzed. Finally, the end mill geometry can be optimized in order to minimizing cutting energy.