Abstract: The apparatus comprises an X-ray source (112), a monochromator (118), a goniometer (170), a position sensitive detector (150), a mechanism to rock or rotate the sample or the X-ray source and computer means (160) for interpreting the data obtained at the position sensitive detector. The method of the present invention includes the steps of generating an X-ray; narrowing the wavelength of the X-ray beam; allowing the particles to diffract the beam; detecting the diffracted beam with a position sensitive detector, collecting the diffraction data from individual particles; rocking or rotating the specimen or the X-ray source for successive times to cover the angular range of reflection of the particles; compilation of the diffraction data in the computer memory to construct the intensity profile for the individual particles; and interpreting the data to determine particle size and particle size distribution.