The nanocrystallized 18Ni maraging steel was plasma nitrocarburized at 460 degrees C for 4 h in a mixture gas of N-2, H-2 and C2H5OH. The surface phase compositions of the specimens were analyzed using X-ray diffractometer. The grain shape and size, and grain boundaries in the subsurface layers of the samples were characterized by electron backscattering diffraction and transmission electron microscopy. The results show that the nitrocarburized layers are composed of alpha-Fe, gamma'-Fe4N and FeN0.049 phases. Most alpha-Fe and gamma'-Fe4N grains show in columnar shape. The major and minor axes of some alpha-Fe grains are elongated and shortened after nitrocarburizing, respectively. In the subsurface layers of the untreated and nitrocarburized specimens, the average areas of gamma'-Fe4N and alpha-Fe grains both with a dimension of nanometer are 0.395 mu m(2) and 0.397 mu m(2), respectively. The gamma'-Fe4N grain boundaries are mainly high angle boundaries with a very small fraction of low angle boundaries. Coincidence site lattice boundaries in the subsurface layer of the untreated specimen are composed mainly of Sigma 3, Sigma 11 and Sigma 13b, and their fraction decreases after nitrocarburizing. (C) 2013 Elsevier B.V. All rights reserved.