Nanocrystalline cementite phase has been synthesized from elemental iron and graphite powder by reaction milling in an efficient dual-drive planetary mill. The strong collision energy field of the dual drive planetary mill reduced the cementite formation time by 60% to 40 h as compared to over 100 h reported in earlier studies. In order to understand the mechanisms of reaction milling, the evolution of particle size, shape, surface areas and phases during different stages of reaction milling were studied. During the initial stage of reaction milling, the ductile powder exhibited flattened morphology with resultant increase in surface area and cold bonding assisted particle size enhancement. The subsequent work hardening of the particle was substantiated from the lattice strain measurement of the milled particle. In the final stage of milling, fracture of the cold worked particles was found to reduce the crystallite size to as low as 100 nm as well as increase the particle sphericity. The decomposition of the metastable cementite phase above 800 degrees C was observed during annealing of milled powder, which was reconfirmed through DTA. (C) 2008 Elsevier B.V. All rights reserved.