This standard specifies the method for determining the particle size distribution of ultrafine powders by using the X-ray small-angle scattering effect. This standard is applicable to the determination of the particle size distribution of ultrafine powders with a particle size in the range of 1 to 300nm, and is also applicable to the determination of the particle size of inorganic, organic sols and biological macromolecules. This standard does not apply to powders whose particle shape deviates too far from the spherical shape, and powders with micropores; nor does it apply to mixed powders composed of particles of different materials.