In order to control cutting force and its direction, a combined milling head was newly developed. This head has two milling cutters which rotate in opposite directions, and a meshing phase of cutters is also variable. The cutting performance of the head was discussed based on the cutting mechanism of a single milling cutter, and a method for simulating the performance was proposed. Cutting force, its change and direction are greatly affected by the change of the ratio of depth of cut for each cutter and the change of the meshing phase. Using the numerical simulation of cutting performance of the head, the optimal cutting conditions, such as the ratio of the up/down cutter's depth of cut and the phase, can be determined. The simulated performance and the experimental one show good agreement.