Thermal management of electronics is becoming demanding with the miniaturization of integrated circuit and the shift to next-generation high-power electronics. Thermal interface material (TIM), which is typically a mixture of silicone oil and high thermal-conductivity fillers, is often used to remove heat from the chip efficiently by eliminating the air gap between the chip and the heat sink/spreader. However, the interface thermal resistance between TIM and chip/heat sink can become significant due to the depletion of fillers in TIM adjacent to the surfaces of chip and heat sink/spreader. The thermal resistance of the filler-depletion layer (FDL) can occupy half of the total contact thermal resistance when the thickness of TIM is on the order of microns, and thus, preventing formation of the FDL is significant. In this work, we experimentally evaluate the impact of FDL between aluminum (Al) surface and TIM by the time-domain thermoreflectance method (TDTR). The Al surface is modified with four kinds of silane coupling agents before depositing the TIM, aiming to reduce the thickness of FDL. These measurements reveal that the increase of thermal resistance near Al surface is not only caused by the depletion of fillers. When the Al surface is hydrophobic, the thermal resistance increases with the increasing water contact angle due to better wettability of silicone oil to Al surface, repelling the fillers way from the surface. On the contrary, when the Al surface is hydrophilic, the thermal resistance decreases with increasing water contact angle. The direct FIB-SEM observation of the interfacial region between Al and TIM reveals that the small particles are more attracted to the hydrophilic Al surface because of larger gain in the surface energy, and smaller fillers reduces the thermal conductivity of TIM.