Highly　pure　Co　nanocrystalline　powders　were　prepared　by　high-energy　ball　milling　under　the　condition　that　all　operations　on　the　powders　were　performed　in　the　glovebox　filled　with　highly　purified　argon　gas.　A　series　of　annealing　experiments　at　different　temperatures　were　carried　out　to　investigate　grain　growth　in　the　milled　powders.　The　as-milled　and　annealed　microstructures　were　observed　and　analyzed　with　transmission　electron　microscopy　(TEM),　high-resolution　TEM,　high-resolution　scanning　electron　microscopy,　and　x-ray　diffraction　methods.　Characteristics　of　the　incontinuous　grain　growth　in　the　milled　nanocrystalline　powders　were　found.　It　is　considered　by　the　authors　that　the　sharp　increase　in　nanograin　size　in　certain　intermediate-temperature　region　is　a　result　of　accelerated　grain　growth　promoted　by　the　stored　energy　as　a　supplied　driving　force,　and　through　a　particular　dominant　mechanism　of　nanograin　rotations　in　contrast　to　grain　boundary　migration　in　polycrystalline　materials.