A nanoindentation induced blistering method has been used to extract the GaN/diamond interfacial toughness (adhesion energy) from four types of GaN-on-diamond samples with varying SiNx interlayer thicknesses. The mode I energy release rate (GIC) was quantified and is presented. Additionally, transient thermoreflectance has been used to measure the thermal boundary resistance (TBR) between the GaN and the diamond substrate. It was found that a thin SiNx interlayer resulted in a lower TBR (15 m2 K GW-1) whilst maintaining a reasonable interfacial toughness (1.4±0.5 J m-2). For interlayers of a similar thickness, samples with a high interfacial toughness and high residual stresses in the GaN had a smaller TBR. This indicates that the intrinsic interfacial characteristics that enhanced the interfacial toughness could be beneficial in improving the TBR.