<p>Solid phase processing, such as friction stir processing, is an advanced manufacturing method that often results in ultrafine grain sizes and superior mechanical properties. The motivation of this study was to demonstrate ultrasonic testing as a nondestructive evaluation method to complement traditional destructive methods for characterizing material microstructure, with an emphasis on grain size determination using a method that may have future applications for real-time inline process monitoring and product validation. The method for measuring grain sizes of polycrystalline metals after solid phase processing was established using ultrasonic shear wave backscattering, building on prior studies on coarse-grained materials. The work involved measuring ultrasonic backscattering for a series of 316L stainless steel specimens with various grain sizes made by friction stir processing, calculating ultrasonic backscattering coefficients from experimental data based on a physical measurement model, measuring ground truth grain sizes of the specimens from electron backscatter diffraction grain boundary images, and building a correlation of ultrasonic backscattering coefficients versus the ground truth grain sizes. The grain sizes of a set of blind test specimens were successfully determined based on the correlation. This work successfully demonstrates the viability of an ultrasonic nondestructive evaluation method for microstructural characterization of material having ultrafine grain structure, as produced by an advanced manufacturing method.</p>