Effects of the composition, texture and pretreatment of cemented carbide substrates on the microstructure of the boundary region between CVD diamond film and the substrate were investigated using a microwave plasma CVD in the CO-H-2 system. Optimum CVD conditions for a uniform coating on to the edge part of cutting insert were : microwave power, 550 W; total pressure, 30 Torr; total flow rate, 200 ml/min; CO concentration, 5-20 vol %; treatment time, 3 5 h. An adherent and tough diamond coating was prepared by initial coating at lower CO concentrations and by subsequent coating at higher CO concentrations. A cemented carbide substrate in the binary WC-Co system which comprised fine-grained tungsten carbide and low content of cobalt was suited for preparation of adherent diamond coating. De-cobaltization pretreatment of the substrate surface in acid solution followed by an ultrasonic microflawing treatment enhanced the nucleation density and adherence of diamond film to the substrate. The rotation of substrate was found to be effective for increasing the uniformity and decreasing the grain size of diamond film.