Samples of pure Cu and a Cu–10% Zn alloy were processed by high-pressure torsion and by high-pressure torsion followed by cold-rolling to a reduction of ∼75%. The grain sizes in these two conditions were measured by transmission electron microscopy and by X-ray diffraction. The experimental results show the average grain size and the width of the grain size distribution are both smaller in the Cu–10% Zn alloy by comparison with pure Cu. This difference is due to the lower stacking fault energy of the Cu–10% Zn alloy. An analysis shows all of the experimental results are consistent with a theoretical model predicting the minimum grain size produced by milling.