Abstract: A simple introduction for the concept and application of FWHM in polycrystal X-ray diffraction was given. From experimental technology, the influence of FWHM measuring made by instrument settings, program settings, sample preparation, data processing and so on was researched. The diffractometer was X′Pert MPD Pro made by Philips, the sample was polycrystal silicon sample provided by Philips. A method how to exactly measure the FWHM was summarized.