High power light emitting diodes (HP-LEDs) have been widely in recent years. However, reliability limited the further cost reduction and light quality improvement of HP-LEDs. In this work, the illuminance distributions and the temperature distribution of the chips were measured to analyze the degradation mechanisms of LEDs under 85 degrees C/500 mA aging. After 240hours aging, the illuminance near electrode what located at the lower left corner of the chip show an obvious decrease. And also the HP-LED devices show a high peak temperature increase and high reverse leakage increase. In addition, the light intensity of the samples showed the light dimmed monotonically with the aging time. In order to find the reason of the failure mechanism of the LED chip, Emission Microscope (EMMI) is used to detect the hot spot in the LED chip. The hot spot position is in corresponding to the peak temperature region. And the current-voltage characteristic curve and voltage-aging time curves indicates that some micro defects are generated in the ohmic contact of chip.