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The grain boundary character distribution (GBCD) of a 100-nm-thick Al thin film was measured as a function of annealing time by transmission electron microscopy-based crystal orientation mapping and compared to a bulk material with a grain size of 23 μm. The most significant difference between the thin film and bulk GBCDs is the concentration of Σ3 boundaries (boundaries with a misorientation of 60° around [111]), which were mostly coherent twins. The length fraction of Σ3 boundaries in the as-deposited thin film is 0.245, more than ten times the length fraction in the bulk sample (0.016). Although the concentrations of Σ3 boundaries are very different in the two samples, the population distributions are strongly correlated for all misorientations except Σ3. The results indicate that the characteristic GBCD develops at grain sizes as small as 109 nm. Annealing the thin film samples at 400 °C for 30 min or more leads to a strong 〈111〉 grain orientation texture and a decrease in the concentration of Σ3 grain boundaries. Grain size distributions for the samples in the current study show good agreement with prior reports that used image-based methods. [ABSTRACT FROM AUTHOR]