The growth mechanism of YBa2Cu3O 7-= thin films grown by the BaF2 post annealing process at low oxygen partial pressure have been studied by transmission electron microscopy. Under the annealing conditions of po2 = 4 Torr and 700°C, BaCuO2 and Y2 Cu2O5 precipitates develop from stoichiometric film precursors of YBCO during annealing. A growth model is proposed based on the observations. In addition, early stage nucleation and growth of both c- and a-axis oriented grains at the substrate interface were observed in quench annealed cross-sectional samples. 90°  symmetrical boundaries form between the a- and c-axis oriented grains. The possible effects of these boundaries are discussed.