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Severe plastic deformation of metals is known to lead to superior properties that cannot be achieved by any traditional metallurgic process. Origin of the superior properties is perceived to be closely associated with grain refinement, a fundamental process during the severe plastic deformation, which is essentially the formation of new grain boundaries. However, the atomistic mechanism of grain boundary formation remains largely obscure. Here, by using in-situ transmission electron microscopy and molecular dynamic simulation, we reveal, for the first time at atomic level, shear-induced low-angle grain boundary (LAGB) formation processes in Au nanocrystal. We discover the LAGB formation is accomplished through inward propagation of nanotwins accompanied by dislocations gliding on twin boundaries, a nanotwin-mediated dislocation slip mechanism, which shows reversible characteristic under reciprocating …