Although densification of alpha-beta-SiAlON ceramics is easier by using sintering additives, it is difficult to achieve the designed composition and therefore the mechanical properties after sintering. This is mainly due to the stability regions of alpha-SiAlONs which are related to cation size of the rare earth. In the present work, alpha-beta-SiAlON starting composition was densified by gas pressure sintefing using oxides of Nd2O3, Y2O3 and equimolar mixture of Nd2O3-Y2O3. The Rietveld technique was used for characterisation of the diffraction pattern and calculation of lattice parameters of alpha-SiAlON. On the basis of obtained data, the stability regions of Y, Nd and Y-Nd doped alpha-SiAlONs were established. The Rietveld results were further confirmed the previous results obtained by classical XRD technique that the stability region for Nd-Y multi-cation doped alpha-SiAlON is larger than Nd- doped system.