The mechanism of optical degradation for the light emitting diode (LED) packages in the light bars under the high-temperature aging condition, one of the accelerated life tests, is studied experimentally and numerically. Some parameters including junction temperature, encapsulant materials, delamination, leadframe surface degradation, and encapsulation are closely investigated. The junction temperature and corresponding thermal resistance are measured by a junction temperature tester. The optical system equipped with a hand-held commercial digital lux meter to capture light brightness and a black box to avoid the light loss and reflection, is for luminous flux measurement. TracePro 5.0 is applied for simulating the light tracing and luminous flux in the optical analysis, while CFdesign 10.0 is used in the thermal simulation in this study. It is found that the lumen maintenance of blue LED packages on the Al substrate start below 70% during the high-temperature aging (with Ta= 120 ℃) after 150 hrs, but after 60 hrs for ones on the PCB substrate. The cause of this is due to the junction temperature of LED package on the PCB substrate higher than that on Al substrate by 11℃. Moreover, it was also found that the white LED packages and blue LED packages without silicone still keeps above 70% at the high-temperature aging until 500 hrs but not for blue ones with silicone. Other parameters such as encapsulant materials, delamination, leadframe surface degradation, and encapsulation have been closely examined. It was found that the encapsulants (with or without phosphors) and interface delamination do not affect the optical degradation during the high-temperature aging, whereas both leadframe surface degradation and encapsulation play important roles on it. In other words, optical degradation of the LED packages was mainly caused by silver chloride which was generated on the leadframe surfaces by exposing silver finish in light and heating conditions after interface delamination happening during the high-temperature aging. Moreover, the blue LED package without silicone (an encapsulant) was found to have better lumen maintenance than one with silicone during the high-temperature aging, due to more light absorption on the darkening leadframe surface through total light reflection for blue LED package with silicone. This paper has presented some important parameters affecting the thermal reliability of the LED packages and provided the clarification of the optical degradation mechanism.