A key issue in using alternative lead-free solder is the evaluation of long-term reliability, such as thermal fatigue. In this paper, an estimation method for the thermal fatigue of Sn-Ag-Cu lead-free solder joints using an inelastic constitutive model was studied. First, the mechanical properties of Sn-Ag-Cu lead-free solders were investigated in order to model the inelastic constitutive equation for finite element analysis. Tensile tests and constant load creep tests were carried out. Next, an inelastic constitutive model was investigated. In the modeling, the inelastic strain rate was decomposed into the steady-state part depending on applied stress and the transient part driven by the difference between applied stress and back stress. Finally, in order to investigate the applicability of the proposed constitutive model for Sn-Ag-Cu lead-free solder, finite element analysis was carried out for the solder joints in a quad flat package. The estimation for the fatigue crack initiation was in good agreement with the actual thermal cycle tests for the quad flat package.