Diamond films grown by chemical vapor deposition have the potential to improve the thermal management and reliability of AlGaN/GaN high electron mobility transistors. The integration of CVD diamond with GaN involves the nucleation and growth of diamond films on GaN which induces a vertical gradient in thermal conductivity of the diamond and can result in bulk properties that depend greatly on growth conditions. Thus accurate characterization of the thermal conductivity of CVD diamond, especially the lower conductivity near the growth interface is needed to assess the impact on AlGaN/GaN HEMTs. In this work, we present measurements of the thickness dependence of CVD diamond with thicknesses ranging from 5 to 13.8 μ m in addition to bulk diamond substrates using time domain thermoreflectance. Measurements were made on the same samples in two different laboratories which showed excellent correlation between the measurements. The diamond properties were then utilized in a thermal model of a 10 finger AlGaN/GaN HEMT to predict the impact of device junction temperature. Compared to a device made on SiC operating at 5 W/mm, a junction temperature reduction of 30-40% was seen when using CVD diamond and the same device size.