Surface generation model of three types of endmills is introduced to analyse the cutting mechanism of an endmill more accurately. Superposition method is introduced to define the effective cusp including the effects of cutter mark. Through the comparison of three endmills, it is shown that the ball-nose endmill is superior to the ball endmill and the flat endmill for inclined milling process in 3-or 5-axis machining modes. By using the objective function minimizing the machining time, appropriate nose radius is selected for various cutter radiuses and cutter inclination angles.