Silicon nitride (Si3N4) is one of the most important advanced ceramics due to its high wear resistance, corrosion resistance, and especially high thermal shock resistance at high temperature. It has been used as cutting-tools, high-temperature bearings and turbochargers for car engines etc. But its applications are still limited by the relatively high Cost Of Si3N4 powders which are now mostly produced by direct nitridation at high temperature for long time with high energy consumption. Self-propagating high-temperature synthesis (SHS) shows potential as an advanced technology for synthesizing Si3N4. One of the reasons of why SHS Si3N4 powder has not been widely used commercially is that most SHS Si3N4 powder reported in literatures are of high P-phase content and SHS Si3N4 with high a-phase content is very difficult to be made, especially for large samples. Since 1990's C.C.GE and his group in Laboratory of Special Ceramics and Powder Metallurgy (LSCPM) has been working on the structure formation mechanism of SHS Si3N4 and the nitridation mechanism of large samples of SHS Si3N4. Recently, great effort has been put on the SHS of homogeneous high a-phase Si3N4. In the present work, the effect of nitrogen pressure and purity, the type and amount of diluents on the a-phase content and microstructure of the product synthesized in a reactor of 10 liter capacity are investigated. Through optimization of processing parameters, SHS Si3N4 powder with high a-phase content up to 96% was synthesized.