The surface nucleation of tension twin in magnesium monocrystalline is investigated by potential energy landscape sampling and dynamic simulations. Surface nucleation via pure-shuffle mechanism is proved without any other pre-existing defects or defect interactions as a trigger event. The stress-dependent reaction pathways and activation potential energies of a tension twin nucleus are reported. The growth of a tension twin nucleus in a submicron-sized sample is simulated by molecular dynamics. During the growth, a twin nucleus with basal-prismatic interfaces as its dominant boundaries evolves into a twin grain which is characterized by coherent twin boundaries. A better understanding of shuffle-induced plasticity in hexagonal close-packed metals is pursued.