Direct diffusion bonding between TC4 titanium alloy and 1Cr18Ni9Ti stainless steel were implemented based on the superplastic diffusion bonding (SDB) method. The joint properties, interface microstructures and the mechanism of superplastic diffusion bonding were analyzed. The results show that for direct superplastic diffusion bonding of TC4 alloy to 1Cr18Ni9Ti stainless steel, the shear strength of TC4/1Cr18Ni9Ti joints is 125.3-148.7 MPa under optimal diffusion bonding conditions including the temperature of 760-820 oC, the bonding pressure of 6-9 MPa and the bonding time of 20-40 min. Compared with the conventional diffusion bonding, the bonding temperature can be reduced by about 100 oC while the shear strength of joints is increased by more than one time. And the bonding specimens have no distinct distortion. In the ultrafine superplastic diffusion bonding of TC4 alloy and 1Cr18Ni9Ti stainless steel, the formation process of the bonded joint can be divided into three stages: close contact forming, activation of contact surface and formation of interfacial metallurgy bonded zone nearby the activation center.