We have measured the low-frequency noises of ultrathin indium tin oxide films to investigate the effect of post annealing on the noise level. The noises obtained obey an approximate 1 / f law in the frequency range f ≈ 0.1-20 Hz. The microstructures and grain sizes of our films were altered by adjusting the annealing conditions. An enhancement of the noise level was observed for those samples comprising smaller grains, where numerous grain boundaries exist. This enhancement in the noise level is ascribed to atomic diffusion along grain boundaries or dynamics of two-level systems near the grain boundaries.