GaN-based high electron mobility transistors have the potential to be widely used in high-power and high-frequency electronics while their maximum output powers are limited by high channel temperature induced by near-junction Joule-heating, which degrades device performance and reliability. Increasing the thermal boundary conductance (TBC) between GaN and SiC will aid in the heat dissipation of GaN-on-SiC power devices, taking advantage of the high thermal conductivity of the SiC substrate. However, a good understanding of the TBC of this technically important interface is still lacking due to the complicated nature of interfacial heat transport. With the AlN being the typical interfacial layer between GaN and SiC, there are issues concerning the quality of the AlN as well as the defects that are contained in the GaN near this growth interface which can impede heat flow. In this work, a lattice-mismatch-insensitive surface activated bonding method is used to bond GaN directly to SiC and thus eliminating the AlN layer altogether. This allows for the direct integration of high quality GaN layers with SiC to create a high thermal boundary conductance interface. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties of the GaN thermal conductivity and GaN-SiC TBC. The measured GaN thermal conductivity is larger than that of GaN grown by molecular-beam epitaxy (MBE) on SiC,showing the impact of reducing the dislocations in the GaN near the interface. High GaN-SiC TBC is observed for the bonded GaN-SiC interfaces, especially for the annealed interface whose TBC (230 MW/m 2 -K) is close to the highest values ever reported. Thus, this method provides the benefit of both a high TBC with higher GaN thermal conductivity near the interface to aid in heat dissipation. To understand the structure-thermal property relation, STEM and EELS are used to characterize the interface structure. The results show that, for the as-bonded sample, there exists an amorphous layer near the interface for the as bonded samples. This amorphous layer is crystallized upon annealing, leading to the high TBC found in our work. Our work not only paves the way for thermal transport across bonded interfaces where bonding and local chemistry are tunable, which will enable and stimulate future study of new theory of interfacial thermal transport mechanism, but also impact real-world applications of semiconductor integration and packaging where thermal dissipation always plays an important role.