Long-term reliability against thermal fatigue is required for solder joints of surface mount IC packages. This study was carried out in order to estimate thermal fatigue strength of the solder joints of the surface mount IC packages. First, a simple elastoplastic analysis of the strain produced in the solder joint was developed and the propriety of the analysis was confirmed by the stiffness measurement of the soldered leads. Next, in order to obtain data used for strength estimation, the torsion fatigue test of bulk solder and solder joint specimens was carried out. Finally, the mechanical and thermal fatigue tests of the solder joints of IC packages were carried out. It was found that the fatigue life of the solder joint by the thermal cycling test can be estimated based on the strain calculated by the present analysis and the mechanical fatigue test data of the solder joint specimens or of the solder joints of the real IC packages.