Plastic deformation of γ'-phase plays an important role on creep of a single-crystal superalloy. Here, we have observe new-typed subgrains of γ' phase formed in regions near the fracture after 1200 °C/ 60 MPa creep. The misorientation of subgrains is within 2° and their misorientation to single-crystal superalloy is directional, which demonstrates that γ'-subgrains result from the rotation of γ'-single crystal via octahedral slip system during rapid plastic deformation. The long-termed plastic deformation accumulation has dominant effect on γ'-subgrains formation. The formation mechanism is then revealed. This findings help us understanding γ'-deformation mechanism and guide to improve its creep resistance.