Because of the self-heating effect of InGaN based high power light-emitting diodes (LEDs), the temperature of the LED chip is much higher than the case temperature (TC). The maximum Tj in our experiment is 148K higher above the case temperature. The voltage drops on p-n junction and equivalent series resistance are extracted from the measured I-V curve, and then the values of equivalent series resistance at different forward power are obtained. Due to the self-heating effect, the values of equivalent series resistance show a very strong forward power-dependent characteristics, the maximum equivalent series resistance reported in this paper is about 1.9Ω at high forward power, being more than twice the minimum value. The power dissipation of equivalent series resistance is increased sharply when the forward power is increased, nearly half of the forward electrical power was dissipated on equivalent series resistance at high power, and it becomes the an important factor restraining the lumen efficiency of high power GaN based light emitting diodes.