Reaction-bonded Si3N4-SiC and Si3N4-bonded ferrosilicon nitride, with Si powder, SiC particles and Fe3Si-Si3N4 particles as raw materials, respectively, are prepared in flame-isolation nitridation shuttle kiln with flowing N-2 at 1723K. There is columnar -Si3N4 in both Si3N4-SiC and Si3N4-bonded ferrosilicon nitride. However, fibrous -Si3N4 is only observed in Si3N4-SiC and Si3N4-bonded ferrosilicon nitride contains much more Si2N2O than Si3N4-SiC. By analyzing the oxidation thermodynamics of Si and Si3N4, it is known that in the process of producing Si3N4-SiC, Si is oxidized first to gaseous SiO and fibrous -Si3N4 is generated with SiO and N-2. The existence of SiO is the reason of low silicon nitridation rate. But in the process of producing Si3N4-bonded ferrosilicon nitride, Si3N4 is easier to be oxidized than Si and Si2N2O is generated on the surface of Si3N4 hexagonal prisms in ferrosilicon nitride particles. Meanwhile, Si in raw materials forms new ferrosilicon alloys with Fe3Si, which decreases the temperature of liquid appearance and blocks some open pores in the samples, which stops the matter loss of nitridation. Liquid ferrosilicon alloys favors -Si3N4 generation from Si direct nitridation and fibrous -Si3N4 transformation, which used to exist in ferrosilicon nitride raw materials.