Grain boundary (GB), as the most common interfacial structure in polycrystalline solids, plays a vital role in the deformation of nanocrystalline metals. Up to date, there are several theoretical models proposed to characterize the GB structures, and to interpret the underlying mechanisms of GB evolution (migration or transformation). However, the exact mechanism of GB evolution at atomic-scale remains elusive due to the technical limits. By performing in-situ transmission electron microscopy study, we successfully made gold bi-crystals with different GB structures, such as symmetrical tilt GB (STGB) and asymmetrical tilt GB (ATGB). By applying shear stresses on the as-fabricated bi-crystals, the sequential atomic-scale observations on the GB evolution were obtained. The results show that STGB migrates without changing its structure while the ATGB gradually transforms into STGB during shearing. Disconnection-mediated mechanisms are proposed to interpret the phenomena. Such observations provide important guideline for exploiting the atomistic mechanisms of interfacial structure evolution.