The lighting advantages become more and more obvious for the white high power light emitting diode (HP-LED) as the next generation lighting source. But the light decay mechanism is complicated because of the compositions of the LED chip, YAG phosphor and even the thermal management of packaging. In order to investigate the light decay mechanism of the white HP-LED, the effects of temperature on the silicone-gel with phosphor, YAG phosphor particles and the silicone-gel are studied, respectively. The optical properties of the blue LED chip are kept in steady state by the HP-LED substrate temperature controlling. The silicone-gel with phosphor, YAG phosphor particles and the silicone-gel is heated by a heating source respectively. The tested results show that the optical radiant power decreases as an exponential relation with the temperature increasing from 27 ℃ to 220 ℃. The thermal properties of the phosphor layer coated on the HP-LED are studied. Results show that the phosphor layer coating can affect the radiant power and the pn junction temperature of the HP-LED. The above results show that the white HP-LED is a heat source interacted both with the LED chip and the YAG phosphor particles.