Abstract: Diamond thin films were deposited on cemented carbide substrates (YG6) by the hot filament chemical vapor deposition(HFCVD). A pretreatment method using etching decarburization by microwave plasma was performed on the substrate. The effect of this pretreatment process on WC substrate surface chemical composition was investigated. The influences of this pretreatment on the adhesion strength and surface morphology of the diamond thin film were analyzed. The cutting performances of the diamond-coated thin film tools were studied by a cutting tests of difficult-to-machining materials. The research results show that the pretreatment process using Ar-H₂ etching decarburization by microwave plasma is an effective method to improve the adhesive strength and cutting performance of diamond-coated tools.