Silicon nitride (Si3N4) was synthesized by a selective combustion reaction of silicon powder with nitrogen in air. The alpha/beta-Si3N4 ratio of the interior product could be tailored by adjusting the Si3N4-diluent content in the reactant mixtures. The synthetic beta-Si3N4 showed a well-crystallized rod-like morphology. Mechanical activation greatly enhanced the reactivity of silicon powder, and the slow oxidation of silicon at the sample surface promoted the combustion reaction in air. The formation mechanism of Si3N4 was analyzed based on a proposed N-2/O-2 diffusion kinetic model, and the calculated result is in good agreement with the experimental phenomenon.