The a-axis oriented ZnO thin films deposited on (011 2) sapphire substrates by pulsed laser deposition were studied to investigate the effects of pre-annealing on oxygen diffusion. The effect was as follows: the oxygen diffusion coefficient decreased, and the oxygen concentration in the tailing regions of the profiles reduced. Ion images of an oxygen tracer revealed the high-diffusivity paths for oxygen tracer diffusion. The temperature dependence of oxygen tracer diffusion coefficients (Db) in as-deposited and pre-annealed thin films were determined to be Db [cm2/s] = 9.2×102 exp (-405 [kJ/mol] / RT) and Db [cm 2/s] = 1.8×103 exp (- 418 [kJ/mol] / RT), respectively. On basis of these results, the crystal orientation on D b and the mechanism for oxygen diffusion were discussed.