In　the　present　study　defect-free　nanocrystalline　(nc)　Ni-Co　alloys　with　the　Co　content　ranging　from　2.4-59.3%　(wt.%)　were　prepared　by　pulse　electrodeposition.　X-ray　diffraction　analysis　shows　that　only　a　single　face-centred　cubic　solid　solution　is　formed　for　each　alloy　and　that　the　grain　size　reduces　monotonically　with　increasing　Co　content,　which　is　consistent　with　transmission　electron　microscopy　(TEM)　observations.　In　the　nc　Ni-Co　alloys,　both　the　ultimate　tensile　strength　and　the　elongation　to　failure　increase　as　the　Co　content　increases.　The　TEM　observations　reveal　that　stress-induced　grain　growth　during　tensile　deformation　is　significantly　suppressed　for　the　nc　Ni-Co　alloys　rich　in　Co　in　sharp　contrast　to　those　poor　in　Co.　We　believe　that　sufficient　solutes　could　effectively　pin　grain　boundaries　making　grain　boundary　motions　(e.g.　grain　boundary　migration　and/or　grain　rotation)　during　deformation　more　difficult.　Thus,　stress-induced　grain　growth　is　greatly　suppressed.　At　the　same　time,　shear　banding　plasticity　instability　is　correspondingly　delayed　leading　to　the　enhanced　ductility.