In the present work the elastic modulus of beta-Si6-xAlxOxN8-x ceramics with 5% of rare earth concentrate has been studied by a dynamic mechanical flexural method. The obtained results are discussed in terms of the coefficient x values, which in this case varied from 0.39 to 1.5, as well as in terms of residual porosity of the sintered samples., In order to follow up the influence of the latter parameter several samples were subjected to post-sintering HIP treatment yielding fully dense material. Elastic modulus for the investigated materials varied from 257 to 312 GPa.