The cubic γ-Si3N4 phase was synthesized by the shock technique from the hexagonal β-Si3N4 phase. The thermal stability of the γ-Si3N4 was investigated during heating in vacuum up to 1773 K. An exothermal heat effect was found at 1690 K and structural investigations revealed complete transformation of γ-Si3N4 to β-Si3N4. Corresponding heat effect value was estimated as 51.3±7.7 KJ/mol. The high-pressure-high-temperature treatment (P=13 GPa, T=1300-2300 K) was applied to γ-Si3N4 to make bulk polycrystalline non-porous samples. It was found that temperatures below 1623 K do not change content of the cubic γ-Si3N4 while temperatures above 2273 K decrease it substantially. Mechanical properties of these bulk samples were measured by acoustic wave and nanoindentation techniques. The maximum values belong to cubic γ-Si3N4: hardness 39-44 GPa, Young’s modulus 475 Gpa and bulk modulus 263 GPa. Equilibrium γ-β line position in P-T phase diagram was estimated by using data obtained in this work. The equilibrium pressure at T=300 K was estimated as P300=7.0±2.0 GPa.