The three point bending fatigue behaviors of WC Co cemented carbides with different binder phase content and composition under cyclic loading at two different stress ratios (R=0.1 and R=0.5) were investigated. The characterization of the fracture surfaces was carried out by using a scanning electron microscope (SEM), and the fatigue mechanism was also studied. The results show that the macroscopic fracture morphology exhibited brittle fracture and toughness fracture. With the increase of Co content, the fatigue sensitivity decreased firstly and then increased. Cemented carbides with complex binder phase (Co + Ni + Cr) exhibited lower fatigue sensitivity than that of pure Co cemented carbides. The fracture mechanism transformed from brittle to ductile at the stress ratio changing from 0.1 to 0.5. With the increase of the stress ratio, the fatigue sensitivity was changed.