A three-dimensional thermal model of LED illuminating device is established by using the finite element method. The natural convective boundaries are applied on the lateral surfaces, and an equivalent convective boundary on the bottom surface is utilized to represent the real effects of heat sinks or fins on LEDs. In this paper, the dielectric layer of LED die bonding is discussed and the influences of external and internal thermal resistances are analyzed. When we enhance the external convective effect, the external thermal resistance is reduced obviously. The thermal conductivity of material and the thickness of dielectric layer are important factors on the internal thermal resistance. We may propose these results to design and develop the global principle for the heat-dissipating package of LED device to increase the performance of LED.