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With the continuing advancement in light-emitting efficiency of high-power light-emitting diodes (LEDs), a nondestructive evaluation on thermal reliability, especially regarding the operating temperature and thermal resistance, which directly affect their lifetime and luminous characteristics, is needed. In this paper, we develop a methodology of using a 3-D numerical simulation for transient thermal-conduction analysis, which is combined with the electrical test method to evaluate the junction temperature of a variety of LEDs. As a result, the simulated temperature for LEDs was found to increase with time, whereas the slug temperature showed the same trend for the simulation and the experiment. The measured results indicate that the thermal resistance of AlGaInP-material-based red and amber LEDs reveals higher variability than that of InGaN-material-based white and green LEDs. Via the luminous flux experiment, we have found that the thermal resistance of the LEDs is not the major determination factor of their luminous flux decay. In contrast, the luminous flux decay of the LED is dependent on the chip material being used. Notably, the slug temperature, observed from the pulsed-driven experiment, of the LED appeared to be lower than that observed in the normal steady operation. It is demonstrated that the lifetime and reliability of high-power LEDs can be improved by decreasing the junction and slug temperatures of the devices. This is important as the slug temperature can be controlled by external cooling techniques. In practice, the developed methodology is not restricted to analyze particular LED packaging forms and can be used to investigate various kinds of multichip LED modules as well.