Abstract: An electronic resonator is employed for the on-line nondestructive measurement of microstructure, as defined by the diameter of grains of the type found in ferritic steel alloys during high rate formation processes. The quality factor (Q) of an inductive-capacitive (LC) tank circuit is determined to obtain a measure of energy dissipated in a steel alloy sample inductively coupled to it. Dissipative energy in the sample results from two sources, hysteresis and eddy currents. Eddy current loss is relatively microstructure insensitive and is mainly related to chemical composition and temperature while hysteresis loss is determined by microstructural factors. If microstructural variables other than grain size, and the factors determining eddy current loss are held constant, grain size becomes the dominant factor in differential energy loss with grain size being inversely related to hysteresis loss.