The deformation behavior of Cu bicrystals with the ∑9（110）{221 } symmetric tilt grain boundary （STGB） under pure shear has been studied by atomistic simulation method with an embedded atom method （EAM） interatomic potential. By using an energy minimization method, it shows that there are two optimized structures of this grain boundary （GB）. The structure with lower energy is the stable one while the other is a metastable structure. The pure shear process of the bicrystals at room temperature has been studied by molecular dynamics （MD） simulation method. The simulated results indicate that there are three structure transformation modes of this GB depending on the shear direction： （1） pure GB sliding; （2） GB atomic shuffling accompanied by lattice dislocation emission from GB; （3） GB migration coupled GB sliding, namely, GB coupling motion. An analysis of the evolution of the GB structure shows that, there are two mechanisms for GB coupling motion depending on the shear direction. One is the collective motion of GB atoms and the other is structure transformation realized by uncorrelated local atomic shuffling processes. The former mechanism can induce transition of the GB between the stable structure and the metastable one, while the latter introduces faceting of the GB. These structure transformation behavior can be well interpreted by employing the coincidence site lattice （CSL） theory.
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