To improve the end-milling efficiency of freeform surface, the feed direction with maximal machining strip width should be chosen at each cutter contact point. Those optimal feed directions usually present a regional regularity overall the freeform surface. However, in most traditional tool path generation methods, the surface is treated as one machining region thus only local optimization might be received. In this paper, the machining strip width tensor of flat-end tool is proposed. Then to get the optimal feed direction the corresponding eignvector of the tensor is calculated. By extracting trisector degenerate points within the tensor field, intersections of adjacent sub-surfaces are selected as starts to construct inside boundaries to divide the surface. Finally, tool paths are generated in each sub-surface separately applying the Iso-scallop method. The proposed method is realized in CATIA CAM platform and tested using one freeform surface. The comparison to several leading existing tool path generation methods is also provided.