Nanocrystalline and ultrafine-grained metallic materials have received considerable attention in recent years due to their ultrahigh strength and unique mechanical properties. However, pure nanocrystalline metals exhibit thermal instability, making them prone to grain growth during thermal treatment and usage, which in turn reduces hardness and strength, and compromises their structural integrity. To improve the thermal stability of these materials, grain boundary segregation of solute elements has emerged as a promising approach. In this study, we used advanced scanning transmission electron microscopy to investigate the grain growth mechanisms of ultrafine-grained Cu-5wt.% Sn annealed at temperatures up to 350°C. Our results indicate that Sn preferentially segregates at general high-angle GBs, while low-angle and coincidence site lattice GBs do not exhibit detectable enrichment of Sn. The segregation at these high angle GBs and GB relaxation explains the increased Vickers hardness of the Cu-Sn alloy during annealing.