GaN high electron mobility transistor (HEMT) played major roles in radar, 5G communication, aerospace and other fields. With increasingly higher output powers developed, however, heat dissipation has become a severely problem limiting the device performance and deteriorating the device reliability and life span. Diamond has the highest thermal conductivity ( > 2 000 W·m - 1·K - 1 ) of the bulk material. Integration diamond film with GaN HEMT can rapidly extract heat from the junction, which may increase the power density. In this paper, the technical advantages and realization approaches of GaN on diamond are stated, and the effect of the thermal boundary resistance on heat dissipation is discussed. The latest researches on the reducing of the thermal boundary resistance are thoroughly reviewed, the difficulties and development in controlling the thermal boundary resistance are also analyzed. It is clarified that under the condition of limited selection of dielectric layer materials, the thermal boundary resistance can be reduced through the enhancing of the quality of the interface between GaN and diamond.