Cobalt　nanocrystalline　powders　with　the　average　grain　size　of　about　17　nm　were　prepared　by　high-energy　mechanical　milling.　Grain　growth　in　highly　pure　and　particle-containing　nanocrystalline　Co　powders　were　investigated　respectively　by　a　series　of　annealing　experiments　at　different　temperatures.　The　characteristics　of　incontinuous　grain　growth　were　found　in　both　the　pure　and　the　particle-containing　nanocrystalline　powders.　It　is　proposed　by　the　authors　that　the　sharp　increase　in　nanograin　size　in　the　transition　between　the　low　and　high　temperature　regions　is　a　result　of　enhanced　grain　growth　promoted　by　the　stored　energy　as　a　supplied　driving　force,　based　on　which　rapid　grain　growth　occurs　through　a　particular　dominant　mechanism　of　nanograin　rotations　in　the　pure　nanocrystalline　powders,　and　that　through　off-pinning　of　grain　boundaries　in　the　particle-containing　nanocrystalline　powders.