Ceramics obtained by spark plasma sintering of powder compositions of Si3N4 was studied by the method of layer-by-layer X-ray diffraction analysis. The effect of carbon diffusion in the surface layers of the sintered ceramics from the graphite mold was observed. The homogeneity of the ceramic phase composition along the depth of the sample was shown. Therefore, a uniform distribution of the temperature field inside the sintered sample was concluded. Keywords: silicon nitride, ceramics, spark plasma sintering, X-ray diffraction.