Abstract: The Si₃N₄ reaction-boned Fe₃Si-Si₃N₄ composites were prepared in an atmosphere of highly pure nitrogen with Si powders and Fe₃Si-Si₃N₄ particles as the raw materials. The phase composition turns out to be Si₃N₄, Fe₂Si, FeSi and Si₂N₂O. The content of Si₂N₂O is about 6% and varies with the section of Si₃N₄ boned Fe₃Si-Si₃N₄ block. By analyzing thermodynamics and microphotography of Si₃N₄, it is determined that Si₂N₂O comes from the oxidation of Si₃N₄. During the sintering, Fe₃Si can transform to a low-melting-point ferrosilicon alloy with part of the Si powder dissolving into it. The molten ferrosilicon alloy, Fe₂Si and FeSi, can fill or block the apparent pores and stop the diffusion of oxygen from central section to the marginal section. As a result, the oxygen reacts with Si₃N₄ of Fe₃Si-Si₃N₄ generating more Si₂N₂O in central section of the block. The oxidation of Si₃N₄ costs the oxygen of the system and allows silicon to reacts with nitrogen directly generating columnar Si₃N₄, not fibrous Si₃N₄, which is completely different with silicon nitrodation mechanism in Si₃N₄ reaction-bonded Si. Meanwhile the existence of Fe α-Si₃N₄ transform to β-Si₃N₄ at 1 450℃.