in machining aluminum alloy that contains a large amount of silicon, highly wear-resistant cutting tools such as diamond coated ones are needed because silicon particles in the matrix of aluminum quickly wear the tools. However, diamond coatings on cutting edges actually debond off easily. In this paper, we examined two possibility in order to improve the adhesion of diamond coatings on cutting edges. One is to optimize the construction of coated drills by changing the shape of cutting edges as well as coating thickness. Another is improvement of the toughness of diamond coatings. By putting together the results obtained, the diamond-coated drills with optimized structure were newly developed. Their performance appeared to be satisfactory in cutting high silicon aluminum alloy.