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The goal of this study is to experimentally and numerically study the thermal and optical behaviors of flip-chip (FC) LED packages with and without underfill. For thermal characterization, the junction temperature (Tj) and thermal resistance (Rth) are evaluated experimentally by a junction temperature tester and an infrared thermal imager, and numerically by ANSYS simulation. After the simulation model validation, the effect of bump number and underfill thermal conductivity on Tj and Rth was further investigated. For optical characterization, the light output of the packages are measured by an integrating sphere system and calculated by TracePro commercial software. Both thermal and optical results for these packages will be presented and discussed in terms of parameters such as number of bumps and underfill in this paper.