Abstract: Silicon nitride (Si3N4) is a series of ceramic material with outstanding technological properties and has been widely investigated for various engineering applications. On the other hand, these kinds of ceramics are of covalent bounding and are very difficult to sinter. This investigation was conducted for understanding the high pressure sintering behavior of Si3N4 micro-powder samples. Sintering was performed at high pressures ranging from 3 to 7 GPa and temperatures in a range of 800~2 000 ℃. The sintering samples are composed of Si3N4 and AlN, Al2O3 and La2O3 additives. The sintering compacts were characterized with the aid of X-ray diffraction, scanning electron microscopy and density measurement methods. The obtained results shows that the conversion of Si3N4 phase is greatly promoted by highly compression and the powder samples could be sintered at quite lower temperatures. Additionally, the high pressure sintering process is observed to be consisting of two stages: at moderate temperatures the sintering process consists of only the joining of the composited particles, while at high enough temperatures, the solid state reactions between the composite powders can be greatly proceeded and resuled in a fully sintered compacts.