The concept of steady state milling time was examined for ball milling of aluminum powder. Four different setups of balls were used while the mill speed and charge ratio were kept fixed. Different criteria (morphology of particles, average particle size, deviation from the average particle size, lattice imperfections and change in crystallographic orientation) were used to study structural evolution of the milled particles and to compare the steady state time for the different milling conditions. Results showed that different criteria may not determine the same steady state time, however, all criteria were consistent in comparing efficiency of the different milling conditions. Moreover, it was found that at a given mill speed and ball to powder ratio (i.e. at a given consumed energy), a change in balls size and filling ratio of vials can improve milling efficiency. Finally, the effect of energy of each impact and the collective energy of all impacts were discussed. (C) 2015 Elsevier B.V. All rights reserved.