Dense and fine-grained beta-Si(3)N(4) ceramics were successfully obtained with a small amount of sintering additives, 1.5 mass% Y(2)O(3) and 0.5 mass% Al(2)O(3), using nanosized amorphous Si(3)N(4) powder by spark plasma sintering at temperatures of 1500 degrees-1800 degrees C and a pressure of 30 MPa under N(2). The beta-Si(3)N(4) ceramics were composed of equiaxed grains with an average size of 300 nm. A higher sintering temperature was required for the densification of submicrometer-sized alpha-Si(3)N(4) powder with the small amount of the additives. The use of nanosized amorphous Si(3)N(4) powder accelerated the densification and the transformation to the beta-phase.