We report on the role of oxygen octahedral tilting in the monoclinic-to-tetragonal phase transition in ultra-thin BiFeO3 films grown on (001) SrTiO3 substrates. Reciprocal space maps clearly show the disappearance of the integer-order Bragg peak splitting associated with the monoclinic phase when the film thickness decreases below 20 unit cells. This monoclinic-to-tetragonal transition is accompanied by the evolution of the half-order diffraction peaks, which reflects untilting of the oxygen octahedra around the  axis, proving that the octahedral tilting is closely correlated with the transition. This structural change is thickness-dependent, and different from a strain-induced transition in the conventional sense. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.