The complex relationships among composition, roartensite start and finish temperatures, morphology of the martensite, residual stress distribution, and quenching conditions produce significant microstructural changes through a carburized case. Variations in the amount of retained austenite, the diffracting-particle size, and hardness were measured every 50μm in depth through a one percent carbon case on AISI-SAE 4320 steel. Measurement were made to a total depth of 2 mm. The percent retained austenite decreases from a maximum of 26% near the surface to a few percent in the bulk. It is shown that the x-ray diffracting-particle size of the martenaite phase is a structure parameter that changes when the martensite morphology goes from plate to lath type. The austenite phase diffracting-particle size is controlled hy the deformatioxis induced by the martensite formation.