The study of the mechanical properties of GaN single crystals can help to solve the problem of cracking in the growth, processing and device applications. In this paper, the elastic modulus and hardness of GaN single crystals with different doping types (undoped, Si-doped and Fe-doped) were tested by nanoindentation method to explore the effect of doping on the mechanical properties of GaN single crystals. The test results show that doping has an important effect on the hardness of GaN single crystals. The hardness of Si-doped and Fe-doped GaN samples are higher than that of undoped sample, this conclusion was also proved by the comparison of heavily doped ammonothermal GaN single crystals. Through high-resolution X-ray diffraction analysis and atomic force microscopy characterization, it is found that factors such as crystal crystalline quality and contact area have less influence on the hardness of GaN single crystals. The nanoindentation slip band length and crystal lattice constant of GaN surface were measured. The results show that, the main reasons for doping affecting the hardness of GaN single crystals are the hindering effect of defects on GaN dislocation multiplication and slip, and the change of GaN lattice constant caused by doping.