The full realization of superior properties of gallium nitride (GaN)-based power devices is limited by the low thermal conductivity of substrates, so the diamond with the highest thermal conductivity becomes an excellent heat dissipation substrate for GaN. Relevant scholars have carried out a lot of research on the combination of high thermal conductivity diamond and GaN devices, including low-temperature bonding technology, substrate transfer technology for directly growing diamond on the back of GaN epitaxial layer, single crystal diamond epitaxial GaN technology and heat dissipation technology for high thermal conductivity diamond passivation layer technology. The reasons of thermal bottleneck for GaN power devices were discussed in detail, and the advantages and disadvantages of these technologies were analyzed and reviewed systematically. The thermal design process development and challenges of various heat dissipation technologies were revealed. The low-temperature bonding technology had the advantages of low temperature and controllable quality of diamond substrate. However, the low-temperature bonding technology had challenges in the high-precision processing and low interface bonding force of large-size diamond films. The diamond directly growing on back of GaN epitaxial layer had excellent interface bonding strength, but technical difficulties were involved such as high temperature, high stress of wafer, and high thermal boundary resistance. The GaN epitaxial technology on single crystal diamond and high thermal conductivity diamond passivation layer technology were limited by the small size of the single crystal diamond wafer, high cost and incompatible process, respectively. Therefore, the development of diamond substrate with low cost and large size, the improvement of stress control of wafer and high interface bonding strength, the reduction of low thermal boundary resistance of GaN/diamond and the enhancement of the performance of GaN-on-diamond devices will be the focus of future development for GaN-on-diamond devices technology.