Ultra-small angle X-ray scattering (USAXS) experiments were conducted with the BL15XU beamline (SPring-8) to perform a particle size distribution analysis of nano-powder, i.e. aggregations of particles with dimensions on the order of nanometers. The samples measured were amorphous nano-SiO2 powders of varying specific surface areas. Since a highly collimated and high-intensity X-ray beam is available, it is possible to obtain USAXS spectra that are only slightly affected by background noise. The USAXS spectra obtained from nano-SiO2 powders were analyzed by a modified Fankuchen technique, a method for particle size distribution analysis adapted by the authors. Using this technique, particle size distribution measurements can be performed over a wide range of particle diameters from 1 to 200 nm. Since this method enables the particle size distribution of nano-powder to be accurately and easily determined, it is expected to be useful in a wide variety of fields. (C) 2004 Published by Elsevier B.V.