To determine the effect of the annealing atmosphere on oxygen diffusion through Ba0.95La0.05FeO3-δ pellets, 18O2 tracer diffusion and high-resolution secondary ion mapping were performed. When annealing in air, the 18O concentration around the surface up to a depth of 40 μm was almost constant. On the other hand, when annealing in vacuum, the 18O concentration obviously decreased. High-resolution secondary ion mapping indicated that the 18O concentration around the grain boundary was reduced. These results suggested that the grain boundary of BLF annealed in vacuum prevents oxygen diffusion.