Silicon–aluminum alloys (Al-Si), with Si contents up to 12% are important materials in automotive and aeronautical industries due to their low density and high wear resistance. The cutting force, surface roughness and tool wear of diamond tools and cemented carbides tool were compared when turning Al-Si alloy. The results indicated that there wasn't remarkable difference between diamond tools and cemented carbides tool in cutting force and surface roughness when rough turning silicon-aluminum alloy. While in finish turning, less cutting force and surface roughness can be obtained when using diamond tools. The flank wears of diamond tools were less than cemented carbides. It was concluded that diamond tools can improve machining characteristics and reduce tool wear.