Cemented carbides are important powder metallurgical tooling materials. Substitution of the traditional cobalt by other cheaper alloy elements is an emerging technology due to low-cost requirements. On the other hand, the fracture toughness of Co-lean cemented carbides should also be improved. The authors recently investigated the effect of using complex-alloy system, inclusing Fe, Co, Cr and Ni elements instead of single Co on the mechanical properties of cemented carbides, and showed possitive improvement of fracture toughness and wear resistance. It indicated that the microstructures of the new binder-WC systmen can be kept in binary phase region on the condition that the carbon content is carefully controlled, at the same time, WC grain sizes in a wide range have been tested for the applicability of new binders. The fracture mechansim, including the fracture toughness in the wake of cracks and their propagation process, was theoratical analysed.