Abstract: The first principles plane wave pseudopotential method based upon density functional theory (DFT) is used to investigate the bonding strength of diamond coating film on cemented carbide substrate with different line defect ratios.The interface molecular models between diamond coating film and substrate are built with different crystal orientations namely[111],[110]and[100], which are used to study the influence of the line defect ratio in cemented carbide substrate on the bonding strength of boundary and best interfacial bonding strength with three different crystal orientations.Resultsshow that the surface energy of cemented carbide substrate increases at first and then decreases as the line defect ratio in substrate surface grows and that when the line defect ratio ρ is 12.5%, the surface energy reaches to the maximum.It is further found that the optimal bonding energy of diamond coating interface with different crystal orientations have different optimal line defect ratios of substrate.The optimal line defect ratio ρ is 6.25%for diamond crystal of[111]and[110]orientations, but ρ=0for[100]orientation.