Abstract: Anand model was used to establish the constitutive equation of Sn3.0Ag0.5Cu solder, and the stress distribution of soldered joints was analyzed with and without underfill.The results indicate that the stress concentrates on the top surface of outermost soldered joint whether the underfill is here or not, and the stress decreases and distributes evenly on the top surface of soldered joint by using of underfill.The Modified Coffin-Mason equation by Engelmaier was utilized to predict the fatigue life of soldered joint;the fatigue life of soldered joint with underfill is longer than that without it. The effects of underfill properties were investigated.The results indicate that the CTE of underfill influences strongly the fatigue life of soldered joints, but the influence of Young's Modulus is little, which will provide a theory guide for practical applications.