Abstract: A new process of diamond coatings was explored with H₂, CH₄ and D₄ as precursors by using microwave plasma chemical vapor deposition technique. The diamond coating containing Si element was deposited on a cemented carbide substrate. This process was attempted to enhance the adhesion of the diamond coating to the substrate. The results reveal that when the flow of D₄ is larger than that of CH₄ the cellular structure is obtained, and the diamond coating with good quality and good adhesion is deposited with a little Si in it only when the flow of D₄ is equivalent to that of CH₄.