This paper analyses the degradation of the optical, electrical, and thermal characteristics of InGaN/GaN-based light-emitting diodes (LEDs) with different chip sizes. Their optical, electrical, and thermal characteristics were periodicity-monitored during the test. The small chip showed an increase in ideality factor and high series resistance. This led to the degradation of the electrical characteristic, and hence to high thermal resistance, finally resulting in low light output power. In the large chip, the high operating current made the junction temperature high. This, together with the thermal degradation on the LED surface and metal pad, led to a decrease in light output power. In the middle chip, the optical and electrical degradation ratio was lower than that in the other samples and, therefore, the light output power decreased to a lesser degree.