Well-separated diamond particles were nucleated and grown by hot filament chemical vapor deposition (HFCVD) onto WC-Co cemented carbide pretreated by Murakami's reagent and H2O2 + H2SO4 solution. The adhesive strength of diamond particles to WC-Co cemented carbide was quantitatively determined in terms of interface toughness by directly applying an external load to the CVD diamond particles. From the measurement of the maximum load required to scratch off the particles, we determined that the adhesive toughness was 14 J/m(2). This value is more than twice as high as that of CVD diamond on smooth silicon substrate and comparable to the cleavage fracture energy of diamond. The newly developed procedure will allow to check the effectiveness of substrate surface pretreatments for further improving the adhesion level of diamond films on WC-Co. (C) 2000 Elsevier Science S.A. All rights reserved.