C-axis-oriented YBa₂Cu₃O_{7- delta} films produced by electron beam and thermal evaporation are composed of mosaic blocks. The deoxygenated YBa₂Cu₃O_x (x=6.1-6.2) films were annealed in an oxygen atmosphere. The change of oxygen concentration in the films during the annealing process was detected by the diffraction peak site of the (0011) peak. Applying the theory of lattice diffusion to analyse the oxygen-in-diffusion in the films, the authors obtain the activation energies for oxygen diffusion in the tetragonal and orthorhombic phases, which are 1.15 and 1.23 eV respectively. The change of shape of (00 11) diffraction peaks indicates that there is a coexistent range of two phases near the transition point. The difference between single crystals and c-axis film is that there are many mosaic boundaries in the c-axis film. A mosaic boundary acts as a short-circuit for oxygen diffusion and makes the diffusion time, during which equilibrium oxygen concentration is attained in the films, decrease greatly.