The study of the structure; mechanical characteristics and high temperature stability in vacuum (using  Pirani – Alterthum technique) and in air (by DTA and TG measurements up to 1600 ^oC) of sintered under high quasihydrostatic pressure (at 4.1 GPa,  4 min heating up to 1800 oC and 4 min holding at 1800 ^oC,) and by hot pressing (at 30 MPa) ZrB₂- and HfB₂ – based materials without and with SiC and Si₃N₄ additions showed that the use of high pressures at comparatively low temperatures and short sintering time makes it possible to obtain mentioned materials with improved mechanical properties as compared to the materials obtained by  other known methods. In the case of SiC-added composites, this is attributed to the diffusion of some C and Si in the ZrB₂ or HfB₂ matrix and Zr and Hf in SiC grains. The Auger and EDS quantitative study and mapping confirm these conclusions. For ZrB₂ and HfB₂ without additives this can be due to high connectivity between grains and a higher density of defects such as twins, dislocations and stacking faults (as TEM study showed).  The stability in vacuum of pure ZrB₂ and HfB₂ occurred to be essentially higher than that of the materials with SiC additions (the beginning of melting of the materials with additions observed at 2150-2160 °C, while the materials without them did not melt up to 2970 °C). The composite material prepared from HfB₂-30 wt.% SiC mixture demonstrated high mechanical characteristics (density r=6.21 g/cm³, microhardness Hv(9.8 N) = 38.1±1.4 GPa, HV(49 N) = 27.7±0.24 GPa,, HV(98 N)=26.3±2.03 and fracture toughness K_1C(9.8 N) = 8.2±0.2, MN•m^0.5, K_1C(49 H)=6.8±0.6 MN•m^0.5, K_1C(98 N) = 6.4 ±0.11 MH•m^0.5) which are essentially higher than that of pure HfB₂ sintered in the same conditions  (r=10.79 g/cm³, Hv(9.8 N)=21.3±0.84 GPa, HV(49 N)=19.3±1.34 GPa,, HV(98 N)=19.2±0.5 and fracture toughness K_1C (49 N)=7.2±0.9 MH•m^0.5, K_1C=5.7 ±0.3 MN•m^0.5, Young modulus E=984 GPa, Poisson ratio µ=0.146). The mechanical characteristics of high pressure sintered material from ZrB₂+20 wt.% SiC (r=5.04 g/cm³, Hv(9.8 N)=24.2±1.0 GPa, HV(49 N)= 17.6±0.7, HV(98 N)=16.73±1.1 GPa, K_1C (49 H)=7.21±1.55 MN•m^0.5, K_1C=6.2 ±1.24 MN•m^0.5, E=386 GPa, µ=0.093) are higher than that sintered by hot pressing at 30 MPa, 1900 ^oC for 1 h  from ZrB₂+30 wt.% SiC (r=5.25 g/cm³, Hv(9.8 N)=22.95±1.0 GPa, K_1C ((9.8 H)=3.44±0.22 MN•m^0.5)..
The availability of large-volume high-pressure apparatuses and the level of technology development make method of high pressure-high temperature sintering promising for industrial application.