Structure of amorphous Fe80B20 alloy was studied after high-pressure torsion method. The deformation leads to nanocrystallization with alpha-Fe particle formation. Although the average size of alpha-Fe particles does not exceed 10 nm, some particles sized of 20 nm were observed as well. These particles consist of alpha-Fe and gamma-Fe regions. The lattices of these regions are in orientation relationship testifying martensite transformation. The martensite transformation occurring in the largest nanocrystals lead to fragmentation of the nanocrystals. (C) 2012 Elsevier B.V. All rights reserved.