p-n junction temperature and thermal resistance of Light Emitting Diode (LED) package are affected by the chip size due to the change of the thermal density and the external quantum efficiency considering the heat dissipation through conduction. In this study, forward voltage was measured for two different size LED chips, 24 mil and 40 mil, which consist constitute 16-chip package. p-n junction temperature and thermal resistance were determined by thermal transient analysis, which were discussed in connection with the electrical characteristics of the LED chip and the structure of the LED package.