In this paper, well-dispersed MnFe2O4 nanocrystals with high saturation magnetization were successfully prepared at a low temperature (below 100 degrees C) by a one step process through microwave-assisted ball milling in aqueous solution. The obtained nanocrystals were characterized by XRD, TEM, VSM. A lattice structure transition at a constant-temperature between tetragonal and cubic Mn3O4 intermediate product phases was observed during the reaction process. By the comparison of the as-milled products obtained with and without microwave-assistance, it was found that the coupling effect of ball-milling and microwave played a key role in the lattice transition and synthesis of MnFe2O4 nanocrystals. (C) 2012 Elsevier B.V. All rights reserved.