Molecular statics simulation was used to study the yielding mechanism of 〈100〉 oriented single crystal copper nanowires under tension. The results show that, the yield of nanowires is initially realised via 111〈112〉 twinning from random activation of partials, and subsequently via slip of 111〈112〉 partial dislocation; in addition, the partial dislocations on different planes successively slip and meet to form stair-rod dislocation at the initial stage of plastic deformation, while some partial dislocations are piled up near the rigid boundary at the later stage of plastic deformation, both of which hinder dislocation slip, resulting in strengthening of the single crystal copper nanowires.