The　influence　of　nanoscale　helium　bubbles　on　the　deformation　twinning　and　detwinning　behavior　of　submicron-sized　Cu　is　investigated　under　tension,　compression,　and　cyclic　loading.　In　situ　nanomechanical　tests　performed　inside　a　transmission　electron　microscope　reveal　that　twinning　and　detwinning　occur　readily　in　helium　irradiated　copper　under　both　tension　and　compression.　Continuous　shearing　of　helium　bubbles　by　Shockley　partials　leads　to　twin　formation,　whereas　the　residual　back-stress　accumulated　from　dislocation-bubble　interactions　assist　in　detwinning.　These　interactions　also　elevate　the　critical　shear　stress　for　partial　dislocation　slip　in　helium　irradiated　Cu　compared　to　that　in　fully　dense　Cu.　The　growth　twin　boundaries　can　significantly　enhance　the　twinning　stress　in　helium　irradiated　Cu　pillar,　and　deformation　twin-growth　twin　boundary　interaction　promotes　the　formation　of　internal　crack　and　thus　accelerates　failure.　The　effect　of　crystallographic　orientation　and　sample　size　on　the　overall　deformation　characteristics　of　helium　irradiated　Cu　is　briefly　discussed.　The　current　studies　show　that　deformation　twinning　and　detwinning　are　also　active　deformation　models　in　helium　irradiated　small-volume　copper.