Single crystal Si was nitrided in a fluidized-bed reactor. The volume fraction of the α/β phases and the nitridation process were studied by X-ray diffraction and electron microscopy. No difference in the α/β ratio and the nitridation rate were found between pure Si single crystal and commercial Si powder including impurities. This indicates that the α/β ratio is not affected by the impurity content. Microstructural observation revealed the internal fracture of Si to several-micron pieces and the formation of Si₃N₄ needles in the early stage of nitridation. Pores were also observed in the fractured Si pieces and the size increased with the progress of nitridation. Aggregation of Si₃N₄ in a band shape and a group of dislocations, forming small angle boundaries, were also observed in the Si piece. The fracture of the Si single crystal appears to be caused by segregation of nitrogen along dislocations, introduced during fluidization, and subsequent formation of Si₃N₄.