Metallized AlN was split up into small pieces with the size of 10mm×10mm×1.0mm, 12mm×12mm×1.0mm and 15mm×15mm×1.0mm, which were then embedded into FR4 materials to fabricate FR4/AlN composites. Then LED module was formed by mounting the same type of LEDs with the composite substrate embedded with the three different sizes of AlN. Then the junction temperature of the three modules was measured with junction temperature tester and semiconductor refrigeration temperature control platform. Meanwhile, thermal simulation was performed on the temperature distribution of the xy plane of FR4/AlN composite. When the ceramic size is 10mm×10mm×1.0mm, 12mm×12mm×1.0mm and 15mm×15mm×1.0mm, the LED junction temperatures is 96.95, 92.94 and 91.81℃, respectively. Simulation results indicate that an increased ceramic size leads to a decreased interface thermal resistance which contributes to a more uniform temperature distribution, resulting in a declined spreading thermal resistance, thus enhancing the overall heat dissipation performance of FR4/AlN composite.