The effect of austenite grain size on kinetics of dynamic ferrite transformation above Ae(3) in a 6Ni-0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic transformation was accelerated. The increase in the fraction of dynamically transformed ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic transformation could be evaluated by an Avrami-type formula. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.