Density functional theory calculations show that graphene flakes with monovacancy at the edge are energetically more stable than the flakes with vacancy in the middle. The energies of metastable and transition states for one step of vacancy motion towards the edge are calculated. We show that thermally activated motion of vacancy towards the edge occurs even at room temperature whereas the probability of return motion back to the middle is negligible. Molecular dynamics simulations of the vacancy motion in graphene flakes confirm this conclusion. The obtained results explain the mechanisms driving structural transformations in graphene. (C) 2012 Elsevier B.V. All rights reserved.