Metal nanoparticles exhibit properties different from bulk bodies due a phenomenon called the size effect which results from their size. At this time, nanoparticles can exhibit outstanding characteristics if the variation in their particle sizes is made uniform. To make this variation uniform, various innovative techniques are used, such as coating around the nanoparticle to prevent aggregation in solution, or dispersing in a polymer. To estimate nanoparticle size and variation when nanoparticles are dispersed in a polymer film, the usual procedure has been to observe particles with transmission electron microscope (TEM) photographs. However, that approach has problems, e.g., it takes time to prepare samples, and observation is on the nanometer scale, so it is impossible to determine overall average information. By using the small-angle X-ray scattering technique, it is possible to estimate the size and variation of nanoparticles in a film, via a few tens of minutes of measurement and analysis, without cutting the sample or performing pre-treatment.