relation between average free path (M) and ductile deformation energy (γ) was proposed, and on the basis of it, a quantitative analysis was conducted for studying the effects of the structural parameters on fracture strength of WC-Co cemented carbides. The results show that, for different WC-Co cemented carbides with different cobalt contents, there exist different critical WC grain size Rc and critical free path of binder Mc. Rc and Mc act as the criteria that determine the growth behavior of crackles. When average free path of cobalt binder M < Mc, or WC grain size R < Rc, crackles will expand mainly across cobalt binder, which will result in intergranular fracture; when M > Mc or R > Rc, transgranular fracture will happen; when M = Mc or R = Rc, concurrence of intergranular fracture and transgranular fracture will take place. Rc and Mc will decrease with increasing of cobalt content, followed by increasing of fracture strength. The dimension of crackles in the circular fissure-breeding district is also a determinative factor to affect fracture strength of cemented carbides.