The critical loads measured in scratch tests have been used in assessing adhesion in film/substrate systems. But critical loads are strongly affected by various parameters such as scratching speed, indenter tip radius, film thickness, etc. Therefore, to assess the interfacial adhesion strength reliably, the stress field developed by a sliding circular indenter was analyzed by considering the effects of friction and plastic deformation of substrate. On the basis of this stress analysis, the elastic deformation energy of the film was estimated by strain matching condition at the film/substrate interface. When the elastic deformation energy stored in the film reaches a critical value, it is released by film failure such as delamination and spallation. Thus, the interfacial adhesion strength can be evaluated in terms of the work of adhesion by extracting the surface energy generated by spallation from the elastic deformation energy. The work of adhesion of PECVD-prepared diamond-like carbon (DLC) films on WC-Co substrates was evaluated. In spite of the distinctly different critical loads measured with the change of scratching speed and film thickness in the same plasma preetched specimen, the comparatively constant work of adhesion was obtained.