Sintered WC-W₂C compacts were used as the substrate for diamond deposition, because the compacts were expected to be a carbon sink during CVD to improve the adhesion strength between diamond film and substrate. The adhesion strength has been found to be improved, as expected, with increasing W₂C content in the range up to about 50%. The diamond deposited WC-(6-22)% W₂C tools show the most excellent cutting performance for hyper-eutectic Al-20mass% Si alloy. The effects of W₂C in the substrate are in detail discussed.