The experiments for the synthesis of the cubic silicon nitride were performed at the dynamic pressure in the range from 20 to 75 GPa in plane container of preservation. The results of the X-ray crystal analysis of the product of the shock compression have showed that the high-pressure cubic phase of the silicon nitride is synthesized. The synthesized phase is the nano-powder, and the size of the coherent scattering regions was about 30 nm. The cubic phase output is increasing both the shock pressure increase and the shock temperature decrease. At the pressure about 53 GPa in the mixture of copper and hexagonal silicon nitride the cubic phase output is up to 95-100%.