Natural　convective　has　significant　effects　on　the　crystal-melt　interface　shape,　temperature　field　and　the　stress　distribution　in　Czochralski　(CZ)　crystal　growth.　In　this　paper,　two-dimensional　numerical　simulations　were　carried　out　to　clarify　the　effect　of　buoyancy　force　on　melt　motion　in　a　CZ　crystal　system.　At　low　Reynolds　numbers,　a　laminar　model　was　used,　while　at　large　Reynolds　numbers,　a　turbulence　model　was　employed.　The　present　paper　investigates　the　natural　convection　from　a　low　Grashof　number　3　×　106　to　a　large　Grashof　number　3　×　1010,　which　corresponding　to　small-size　CZ　system　to　large-size　CZ　system.　The　numerical　results　show　that　the　flow　state　is　not　only　related　to　Grashof　number　but　also　the　ratio　of　the　melt　height　to　the　crucible　radius.　When　Gr　is　large　than　a　critical　value　108,　the　flow　becomes　unstable,　the　laminar　model　is　no　longer　suitable,　and　it　is　found　that　with　the　increase　of　Grashof　number,　the　turbulence　flow　and　intensity　becomes　stronger　and　temperature　gradient　near　the　interface　becomes　sharper,　which　is　harmful　to　the　crystal　growth.