In this article, a directly proportional relation between average free path (M) and ductile deformation energy (y) 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 R-c and critical free path of binder M-c. R-c and M-c act as the criteria that determine the growth behavior of crackles. When average free path of cobalt binder M < M-c, or WC grain size R < R-c, crackles will expand mainly across cobalt binder, which will result in intergranular fracture; when M > M-c or R > R-c, transgranular fracture will happen; when M = M-c or R = R-c, concurrence of intergranular fracture and transgranular fracture will take place. R-c and M-c 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.