Fe(3)O(4) nanoparticles coated with an oleic acid as a surfactant were reduced and nitrided by hydrogen plasma and nitrogen-hydrogen mixed plasma generated by 13.56 MHz Radio-Frequency. Fe(3)O(4) nanoparticles could be reduced to alpha-Fe nanoparticles with small average grain size about 35 nm and with little carbonization by hydrogen plasma treatment. In the nitrogen-hydrogen mixed plasma treatment, it was clarified that the mixture of hydrogen with ratio of 10-30% to nitrogen accelerated nitridation of a-Fe nanoparticles. This effect was derived by generating exited species with higher activity such as NH ions and its radicals. Furthermore, it was also revealed that nitridation behavior strongly depended on average grain size of as-prepared a-Fe nanoparticles. In the plasma nitridation of a-Fe nanoparticles with average grain size of 35 nm, the nitridation was easily promoted with lower input power. This could be attributed to large surface energy of nanoparticles because nanoparticles had wide surface area than bulk materials. (C) 2011 Elsevier B.V. All rights reserved.