In the present work, diamond thin films were deposited on the cobalt-cemented tungsten carbide (WC-6%Co and WC-3%Co) by Hot Filament Chemical Vapor Deposition (HFCVD). After sythetically analysing the mechanism of adhesion on the system of hard film and soft substrate, an investigation had been made on the methods of improving the adhesion by studying the effects of Co content, surface etching treatment and process parameter in diamond nucleation and growth. Conclusion can be drawn as following:(1) One-step and Two-step chemical etching methods can remove the Co contents on the cemented carbides substrate and prevent Co negative effects in the process of deposition. Compared with one-step method, two-step etching is a more effective method for removing cobalt from WC-Co. This role can decrease the diffusion of Co to substrate surface and guarantee the deposition of good qualitative and adhesive diamond films.(2) Among two-step etching, firstly using etching reagent B to attack WC grain 30 minute and then with etching reagent C 30 second removing Co is the most effective way consequently. The Co content of surface area can be reduced to 0.82% and 1.36% for YG6 and YG3 sample, respectively. The nucleation density can increase remarkably, at the same time, the purity of diamond thin films can improve greatly. A slight preference towards {110} oriented texture can be observed by XRD analysis.(3) The adhesion between the diamond thin films and the cemented carbides substrates is dependent on the content of cobalt at the surface, the WC-Co substrate properties, the nucleation density and the quality of diamond films. Especially, the content of cobalt is a more important factor. The decreasing of Co content is easy for improving the adhesion between substrate and thin film. Of the two types of substrate used, WC-6%Co and WC-3%Co, it was easier to remove the Co from the former 6% samples.(4) The conditions of diamond film deposition evidently affect on the nucleation density, the morphology, the structure, the purity of diamond film and lastly its adhesion strength. Our experiments show that the adhesion between diamond thin films and cemented carbide substrates is strong when substrate temperature is 750, chamber pressure is 20Torr and methane concentration is 1%.