The XRD and TEM techniques were used to study the structural evolution in the mechanical alloying and annealing of Fe42.5Al42.5Ti5B10. The mechanical alloying mechanism during ball milling and the grain growth mechanism during annealing of the powder are also discussed. The results show that the diffusion of Al，Ti and B atoms into Fe lattice occurs during milling，leading to the formation of Fe(Al，Ti，B) solid solution. The process is controlled by continuous diffusion mixing mechanism. The alloying reaction is completed after 50h and the final product is a powder composed of nanocrystalline Fe(Al，Ti，B). Besides the relaxation of crystal defaults and lattice stress，the decomposition of Fe(Al，Ti，B) occurs to form FeAl and TiB2 during heat treatment of the 50h milled powder. The activation energy for the nanocrystalline FeAl growth was calculated to be 525.6kJ/mol according to kinetics theory of nanocrystalline growth.