The effect of sintering additives on superplastic deformation of nano-sized beta-Si3N4 ceramics has been studied by compression tests at 1500 degrees C. The sintering additives were (i) Y2O3+Al2O3; (ii) Y2O3+MgO; and (iii) Y2O3. Nano-sized Si3N4 ceramics with different sintering additives had similar microstructures. For the first two sintering additives, the stress exponents were determined to be similar to 2 at a lower stress region and similar to 1 at a higher stress region, where the strain rate was dependent on sintering additives only at the higher stress region, and was independent at the lower stress region. Nano-ceramics with Y2O3 additives had only one region, which had a stress exponent of similar to 1 within the stress range that we studied. The results could be explained by the different deformation mechanisms at the higher and lower stress regions and the influence of viscosity of liquid phase on the transition stress.