By adopting in situ nano-mechanical testing in transmission electron microscope, we reveal that nanoscale helium bubbles can simultaneously enhance strength and ductility in small-volume single-crystal Al-4Cu pillars, which breaks the paradox of trade-off between strength and ductility. Nanoscale helium bubbles serve as internal dislocation sources and shearable obstacles, which promote dislocation nucleation and storage, and give rise to higher flow stress, controllable plasticity and larger uniform deformation in Al-4Cu pillars. Nanoscale helium bubbles are rather stable under plastic straining. Bubble coarsening and coalescence are only observed at final localized deformation regime. Display omitted