Recently, high power LED is more and more important due to requirement of illumination. To achieve the greater performance of electric path and heat dissipation ability, the metal core PCB (MCPCB) is used to replace the common FR4 PCB. The MCPCB is composed of copper film, dielectric and metal base. Metal base is made of copper or aluminum and has high thermal conductivity to conduct heat from LED to heat sink. However, the low thermal conductivity of dielectric layer in the middle of MCPCB will cause the temperature raise. In this study, we use 30% diamond powder as the filler of dielectric layer and combined the PCB and flat plate heat pipe (PCB-FPHP) without metal base to enhance the effective thermal conductivity. In order to verify the performance of diamond powder PCB-FPHP, we experimental measured the PCB-FPHP and common MCPCB conducted with 3 different materials include Cu, Al and FPHP by IR camera. The experimental results shows that the thermal resistance of PCB-FPHP is 36%, 78% and 86% lower than MCPCB-FPHP, MCPCB-Cu, and MCPCB-Al respectively. It is obviously that the thermal resistance of metal base and the thermal grease cannot be neglected especially in high power application. The structure of PCB-substrate we propose in this paper can reduce the junction temperature about 0.36°C/W compare with MCPCB-substrate. It is a meaningful number for high power LED application.