Abstract: A method and apparatus for the "on-line", nondestructive measurement and control of grain size in various materials, such as steel, powders, ceramics, semiconductors, and crystalline polymers. A substantially monochromatic beam of X-rays is directed at the material as it moves relative to a predetermined measuring station. The incident X-ray beam has a predetermined cross-sectional area so that over a plurality of time intervals a plurality of corresponding non-overlapping area segments are irradiated. The time-integrated intensity of diffracted radiation from each non-overlapping area segment is measured and the relative variance of the intensity measurements within the total area irradiated is determined. The relative variance of the measured intensities is then used to determine the grain size of the products which is generally reported as the mean grain diameter, weighted with respect to volume.