This paper deals with the reaction-bonded silicon nitride I terms of its microstructural development during nitrida-tion. Silicon powder compacts were reacted with nitrogen at 1185$^{circ}C$ and 13$65^{circ}C$ according to the nitriding schedule. Microstructures of nitrided specimens were examined by means of optical and scanning electron microscope to discuss the nitridation or microstructural development at initial and intermediat stage of nitridation. Reaction products were also analysed by X-ray diffraction method at each stage of nitridation. The results indicate that ho-mogeneous and uniform microstructure with find porosity can be obtained only under the reaction condition. such as slow and relatively constant reaction rate with time.