Revealing　the　atomic-scale　deformation　mechanisms　of　metallic　nanowires　(NWs)　is　important　for　their　practical　application.　However,　there　are　few　reports　providing　direct　atomic-scale　experimental　elucidation　on　those　metallic　NWs.　Here,　we　conduct　serial　in　situ　deformation　tests　on　silver　(Ag)　nanowires　with　diameters　of　3-11　nm.　The　in　situ　atomic-scale　observations　reveal　a　transition　in　the　deformation　mechanism　with　a　decrease　in　the　diameter　of　Ag　NWs.　For　the　[5　(5)　over　bar4]　and　[001]　oriented　NWs　with　diameters　of　similar　to　11　nm,　the　plastic　deformation　is　dominated　by　full　dislocation　that　involves　leading　and　trailing　partial　dislocations,　whereas　the　full　or　extended　dislocations　are　rarely　observed　in　the　NWs　with　diameters　in　the　range　of　similar　to　5-8　nm,　and　their　plastic　deformation　is　governed　by　SF　generation　and　annihilation.　Moreover,　for　the　[(1)　over　bar　11]　oriented　NWs,　60　degrees　mixed　and　pure　edge　dislocations　are　frequently　observed　when　the　diameter　is　approximately　5　nm　and　the　plastic　deformation　is　accommodated　by　relative　slip　between　two　adjacent　{111}　planes　for　NWs　with　diameters　below　similar　to　3　nm.　These　results　indicate　that　the　plastic　deformation　not　only　depends　on　the　size　of　NWs　but　also　can　be　significantly　impacted　by　the　loading　orientation.　(C)　2016　Elsevier　B.V.　All　rights　reserved.