Pure copper (99.97%) samples with varied grain sizes were cold rolled and annealed at first and then their grain boundary character distributions (GBCDs) were characterized by electron back scatter diffraction (EBSD) techniques. The results showed that the sample with finer initial grain size (12 μm) appeared to be largely populated by the so-called special boundaries such as Σ3ⁿ (n=1, 2, 3) after cold rolling and annealing, the fraction of Σ3ⁿ boundaries reached 75.7% and the averaged size of clusters of grains with Σ3ⁿ (n=1, 2, 3) orientation relationships (Σ3ⁿ CG) were as large as 200 $\mu$m in this sample. However, with initial grain size increasing, the fraction of special grain boundaries as well as the size of Σ3ⁿ CG in the samples were decreasing dramatically. In situ EBSD observation, twin-filtering and five parameter method (FPM) analyses indicated that triple-junctions are the preferred sites for the nucleation of incoherent Σ3 boundaries which play a important role in enhancing the fraction of special boundaries.It accounts for primarily the desired GBCD results as obtained in the sample with finer initial grain size.