Twins play an important role in the deformation of nanocrystalline (NC) metals. The size effects of {10 (1) over bar2} tensile/{10 (1) over bar1} compressive lamellar twins on the tensile strength and deformation mechanisms of NC hcp cobalt have been investigated by a series of large-scale molecular dynamics simulations. Unlike the size effects of twins on the strength for polycrystalline fcc metals, the strength of NC hcp cobalt with lamellar tensile/compressive twins monotonically increases with decreasing twin boundary spacing (TBS) and no softening stage is observed, which is due to the consistent deformation mechanisms no matter TBS is large or small. These consistent deformation mechanisms can be categorized into four types of strengthening mechanisms: (i) Partial basal dislocations nucleated from grain boundaries (GBs) or twin boundaries (TBs) intersecting with TBs/GBs; (ii) Phase transformation from hcp to fcc; (iii) < c + a> partial edge dislocations nucleated from TBs intersecting with basal partial dislocations; (iv) Growth of the newly formed secondary tensile twins inside the primary compressive/tensile twins. The observed multiple twinning in MD simulations has also been confirmed by TEM after tensile testing in NC cobalt processed by severe plastic deformation.