Abstract: The composition and structure of ferro-silicon nitride prepared via flashing combustion and the starting material FeSi75 were analyzed by SEM and EDS. The formation mechanism of Fe_xSi particles in ferro-silicon nitride was studied in combination with thermodynamic analysis and flashing combustion. The results show that nitridation of metallic silicon and the silicon part of ξ phase (FeSi2.3) form silicon nitride during the synthesis of ferro-silicon nitride with FeSi75 of 74 μm. Fe_xSi particles in ferro-silicon nitride come from nitridation of ξ phase. When the molar fraction of[Si] in ξ phase decreases to 25%, the activity of[Si] is approaching to zero and the nitridation reaction tends to balance. Fe_xSi particles form from the residual part of ξ phase which can not be nitrided anymore. The atom proportion of Fe to Si is 3 to 1. The size of Fe_xSi particles and homogeneity distribution relate to the particle size of phase.