High pressure torsion (HPT) is one of the most widely used severe plastic deformation methods which enable to obtain a crack-free ultra-fine grained bulk sample. In the process, a disk-shaped sample is pressed between two anvils and deformed via surface friction forces by rotating one of the anvils. The aim of this study is to nondestructively characterize the variations in the deformation uniformity of the severely deformed disks. Various sets of low carbon steel samples were obtained by applying the constrained HPT process up to 6 turns. Magnetic Barkhausen noise (MBN) method was used in order to evaluate the samples in a nondestructive manner via a commercial system. The results were compared with those of metallographic examination, hardness measurements and X-ray diffraction (XRD). The initial stages of HPT revealed the effects of conventional plastic deformation on MBN; however with further straining, grain refinement caused increase in MBN signals. (C) 2009 Elsevier B.V. All rights reserved.