Thermal fatigue properties of solder joints encapsulated with underfill were studied conducting thermal shock tests. Flip chip package with electroless nickel-immersion gold plated on FR-4 substrate and the Sn-3.0Ag-0.5Cu solder ball was used. The fatigue property of package with underfill was better than the package without it. The fatigue property of package with underfill which has a higher glass transition temperature (Tg) and lower coefficient of thermal expansion (CTE) was better than that of package with underfill with lower Tg and higher CTE.