GaN semiconductor is characterized by strong bonding and high vapor pressure of nitrogen. The strong bonds limit an efficiency of annealing procedures required for variety of semiconductor technologies, for example, post-implantation annealing, or doping by diffussion. Maximum temperatures employed up to now (at ambient pressure) in GaN annealing have not exceeded about 1100°C. A desired increase of annealing temperatures would cause a decomposition of GaN unless an elevated pressure of N2 is supplied. In this work, we report onapplication of high pressure annealing procedures (temperatures up to 1550°C and pressures up to 16 kbar) which enabled us to study variation of the properties of epitaxial films and bulk crystal of GaN. In particular, we discuss the following results obtained using high pressure annealing: i) structural quality improvement and increase of the thermal strain in as grown epitaxial layers of GaN/A12O3, The annealing at above 1300°C resulted in the decrease of the X-ray rocking curve width from about 700 arc sec. down to 470 arc sec., ii) drastic increase of bandedge (bound exciton) photoluminescence intensity iii) enhancement in removal of implant damage, iv) increase of diffusivity of Zn and Mg atoms (introduced by implantation and/or diffusion from external source). For Zn in epitaxial layers of GaN/A12O3 a diffusion starts at 1200–1250°C, v) enhancement of the blue-photoluminescence intensity in Zn and Mg implanted GaN. The performed experiments give an evidence of the importance of the defect (dislocations) in diffusion of Zn and Mg in the GaN semiconductor.