The work aims to improve adhesion of diamond films on cemented carbides. The diamond films were deposited on cemented carbide substrates in the method of hot filament CVD. The effects of four pretreatment methods, i.e., sandblasting plus one-step etching, sandblasting plus two-steps etching, AlCrN transition layer and traditional two-steps etching method, on quality and adhesion properties of the diamond films were investigated. Morphology and roughness of the pretreated cemented carbide substrates were analyzed. Morphology, structure and properties of the diamond films were characterized with field emission scanning electron microscopy (FESEM), X-ray diffractometer (XRD), Raman spectroscopy and Rockwell hardness tester. Sandblasting was beneficial to uniformly distributed dimples on the substrates, and nucleation density and uniformity of diamond, and especially agglomeration of diamond particles. Although the surface roughness of AlCrN transition layer was not high, a large number of convex particles provided excellent nucleation sites. Nucleation density of diamond treated in this way was superior to that treated in traditional two-steps method to some extent. Provided with the same deposition parameters of diamond films, the adhesion of diamond films pretreated in the methods of two-steps etching, sandblasting plus one-step etching and sandblasting plus two-steps etching was class HF4, HF3 and HF1, respectively, while the adhesion of AlCrN transition layer was poor. AlCrN transition layer can prevent outward diffusion of Co and improve purity of diamond, but the diamond films exhibit high internal stress and poor adhesion. The combined pretreatment of sandblasting and etching not only improve crystalline quality and purity of diamond, but also enhance adhesion of diamond films.