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This paper designs a packaging structural of six-chip white LED down light with a total power of 15 W. The highest temperature of LED chip is 110.5°C, and the temperature of heat sink is 71.6°C ~ 73.2°C by finite element method (FEM). The experimental data are obtained by the thermocouple, with forward current of 700-mA at the ambient temperature of 35°C. The relative error between the simulation results and experiment calculation is merely 2.3%. The increased electrical currents used to drive the LED have focused more attention on the thermal paths. Effect of chip packages on junction to board thermal resistance is compared for both Al2O3 ceramic substrate and YLiO2-Doped AlN Ceramics substrate (170 W/m°C after sintering at 1600°C for 6 h). The higher thermal conductivity of the AlN provided about 7.5 times better thermal performance than the former. The optimization dimension of heat sink and influence of substrate materials, rosin joint, and ambient temperature on the temperature distribution are also researched. The heat dissipation will be enforced efficiently after these optimal methods.