A new method of quantitative evaluation for the toughness of interface between thick diamond films and Co-cemented tungsten carbide (WC-Co) substrates is reported. This new method derives from the previously developed technique, which was applied for the quantitative evaluation of the adhesive toughness of discrete diamond crystallites on Co-cemented tungsten carbide (WC-Co) substrates [S. Kamiya et al., Diamond Relat. Mater. 9 (2000) 191-194] and has been applied here to evaluate the adhesive toughness of various diamond films on WC-Co cutting inserts. The adhesive toughness was successfully obtained to be in the 15-1.3 J/m(2) range and was depending on the morphology of the interface. (C) 2002 Elsevier Science B.V. All rights reserved.