A commercial silicon nitride powder with sintering additives was ground by high-energy milling to reduce particle size. Nanometer sized powder was obtained. The powder was densified for short time by spark plasma sintering to prevent grain growth. Nanometer-grained Si3N4 ceramics were obtained. Plastic deformation of the Si3N4 nano-ceramics has been studied in compression over a wide range of strain rates and temperatures. The experimental results revealed that a transition in stress exponent, n, at each temperature. The n value decreased from ~ 2 to ~ 1 with increasing applied stress. Activation energy was also different for the two regions, decreasing from 858.2 kJ/mol in the n ~ 2 region to 571.8 kJ/mol in the n ~ 1 region. Effect of sintering additives on plastic deformation was also discussed.