Microstructural characteristics of extra low carbon lath martensite after cold rolling to 85% reduction and equal-channel angular pressing of four passes were studied by means of X-ray diffraction (XRD) peak profile analysis and transmission electron microscopy. The modified Willilamson-Hall and Warren-Averbach methodologies were implemented to determine crystallite size distribution, distribution density and character of prevailing dislocations of the deformed structures. Cold-rolled steel showed lamellar dislocation boundaries which subsequently disappear at large strain by equal-channel angular pressing, turning to a highly and uniformly dislocated structure involving equiaxed ultrafine grains. XRD peak profile analysis revealed that upon further straining of lath martensite; i) crystallite size decreases, ii) dislocation density increases slightly and iii) fraction of screw dislocations increases. (C) 2008 Elsevier B.V. All rights reserved.