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GaN-on-diamond device cooling can be enhanced by reducing the effective thermal boundary resistance (TBR eff ) of the GaN/diamond interface. The thermal properties of this interface and of the polycrystalline diamond grown onto GaN using SiN and AlN barrier layers as well as without any barrier layer under different growth conditions are investigated and systematically compared for the first time. TBR eff values are correlated with transmission electron microscopy analysis, showing that the lowest reported TBR eff (∼6.5 m 2 K/GW) is obtained by using ultrathin SiN barrier layers with a smooth interface formed, whereas the direct growth of diamond onto GaN results in one to two orders of magnitude higher TBR eff due to the formation of a rough interface. AlN barrier layers can produce a TBR eff as low as SiN barrier layers in some cases; however, their TBR eff are rather dependent on growth conditions. We also observe a decreasing diamond thermal resistance with increasing growth temperature.