Nanodisperse silicon nitride powders were produced by different methods of synthesis. The effect of the production routes on the grain-size distribution and the dislocation density in the powders were studied by high-resolution X-ray diffraction. The average grain-size and the dislocation density of the samples were determined by the recently developed modified Williamson-Hall and Warren-Averbach procedures from X-ray diffraction profiles. A new numerical method provided log-normal grain-size distributions from the size parameters derived from X-ray diffraction profiles. It was established that the powder produced by silicon nitridation and milling had lower average grain-size and wider size distribution than the sample crystallized from amorphous silicon nitride powder synthesized in plasma reactor. The grain-size distribution and the area-weighted average grain-size obtained by X-rays were in good agreement with those determined by TEM and from the specific surface area, respectively. The dislocation density was found to be between 10(14) and 10(15)m(-2).