In this work, Si3N4 powders containing similar to 88% by weight of -phase were prepared by adopting a facile yet efficient combustion synthesis strategy under a nitrogen pressure of 6MPa. A unique radial-spheroidal cluster morphology was strikingly observed in the as-fabricated Si3N4 products. More significantly, a three-step growth mechanism was rationally proposed herein based on quenching experiments and thermo-kinetic calculations. As concluded, both the combustion temperature achieved and the sustaining time in high-temperature period played significant roles in obtaining silicon nitride powders with controlled high contents of -phase Si3N4.