Abstract: The plane strain fracture toughness K_IC of WC-20 (Fe/Co/Ni) cemented carbides was obtained on single-edge notched specimen machined by electric discharge usiny extrapolation. The results showed a linear correlation between appearance strain energy release rate G_Q(ρ) and curvature radius ρ of the specimen notch, i. e. G_Q(ρ)=G_IC +Aρ. By observation of fracture morphology using scanning electionic microscope, 4 kinds of fracture modes were considered:ductile rupture in binder phase,WC grain cleavage,intergranular fracturee along WC grain boundary and fracture near the interface of WC/binder. The fracture process of various fracture modes has been discussed.