Natural convection in the melt phase of the Czochralski crystal growth system is unavoidable due to the gravitational field. Both horizontal and vertical temperature gradients influence the stability of the melt convection. For a tall crucible, a boundary layer type of convection is dominant, while Rayleigh-Benard-type convection occurs for a shallow crucible. A low Reynolds number kappa-epsilon model was employed, but found to be inadequate for prediction and a more sophisticated model is required.