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In this paper, we present the thermal, electrical, and optical analyses of light emitting diode (LED) packages with different chip sizes. The LED packages under investigation employed the same configuration of package components, except for the chip sizes. The forward current was found to increase with the chip size at the same forward voltage due to the area increase of current spreading. The luminous flux and optical power were found to increase with the chip size at the same current density. The thermal analysis was made by the transient thermal measurement and thermal simulation using the finite volume method. It was demonstrated that the thermal resistance decreased with the chip size under the same package conditions both by simulation and experiment. The bulk thermal resistance and spreading thermal resistance were combined together to give out a quantitative investigation of the partial thermal resistance variation. Moreover, the spreading thermal resistance was found to have a great effect on the total thermal resistance of LED packages.