In　this　paper,　the　mechanical　behavior　of　Al,　Ni,　Cu,　Au　and　Ag　nanowires　(NWs)　with　a　coherent　twin　boundary　parallel　to　their　longitudinal　axis　was　investigated　using　molecular　dynamics　simulation.　Our　results　show　that　the　twin-structure　NWs　with　circular　cross-section　exhibited　a　strong　twin　thickness　strengthening　effect,　which　led　to　their　yield　stress　continuing　to　increase　as　the　twin　thickness　decreased.　The　plastic　deformation　mechanisms　of　these　NWs　were　significantly　affected　by　both　gamma(sf)/gamma(usf)　[the　ratio　between　the　stacking　fault　energy　(gamma(sf))　and　the　unstable　stacking　fault　energy　(gamma(usf))]　and　the　twin　thickness.　As　the　twin　thickness　decreased　to　three　atomic　layers,　we　discovered　two　new　deformation　models　in　the　NWs.　For　Al　and　Ni,　their　plasticity　was　accommodated　by　lattice　distortion　resulting　in　brittle-like　necking.　Meanwhile,　for　Cu,　Au　and　Ag,　their　plasticity　was　accommodated　by　lattice　distortion　and　rearrangement　that　led　to　new　grain　formation　in　the　NWs.