Heat treated die steels are durable and resistant to abrasion. However, machining them is not very efficient. To improve the machinability using the end-milling process for high hardness die steels, we proposed an end-mill shape through analysis of the cutting force and simulation. In this study, we determined the important factors affecting the cutting force among several elements of end-mill shape using the customized cutting simulator and the design of experiments (DOE) technique. After the selecting the effective factors based on the simulation and DOE results, various end-mills were fabricated by adjusting the parameters. In the experiment, the cutting force between 1 pass and 40 pass were measured and the average value compared with each end-mill shape. Edge radius, radial relief angle and axial relief angle were selected as a key parameters and optimized by measuring the cutting force through repeated and well controlled experiments. In conclusion, the effective factors were confirmed and we could now determine the optimum shape of end-mill to minimize the cutting force for high hardness die steels.