Silicon nitride powder compacts without additives were produced by underwater-shock wave. The as-compacted specimens and sintered ones at various temperatures were characterized by relative density, micro Vickers hardness and fracture toughness. Microstructural characterization was also made by SEM and TEM. The relative density of an as-compacted specimen was about 75%, but hardness value was quite low about 2GPa. Mechanical properties were remarkably improved by sintering. Hardness and fracture toughness values of the specimen sintered at 1973K for 5h were about 24GPa and about 4MPa√m, respectively. TEM observation of the compact sintered at 1973K showed that there was no large glass pockets at grain boundary edges and that the glass phases at grain boundary surfaces were a few atoms thick.