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The goal of this paper is to experimentally and numerically study the thermal behaviors of flip-chip (FC) light-emitting diode (LED) packages with and without underfills and, furthermore, to compare it with conventional wire-bonding LED packages in order to understand its thermal dissipation mechanism. In experimental analyses, the junction temperature Tj and surface temperatures are measured by a junction temperature tester, thermal couples, and an infrared thermal imager, whereas the numerical analysis is carried out by an ANSYS simulation. After the validation of the simulation model with experimental results, the effects of FC bump number and underfill thermal conductivity on both Tj and thermal resistance Rth of the packages are investigated by this validated model. Furthermore, a simple model of effective thermal conductivity for a composite material of bumps and underfills is proposed for thermal analysis of the FCLED packages. Thermal results for the packages are presented and discussed in terms of volume fraction and thermal conductivity for bumps and underfills in this paper.