We have carried out a numerical simulation of the hydrothermal process for growing bulk single crystals of ZnO using SC/Tetra as the computational fluid dynamics software. The autoclave is assumed to be axisymmetric. The heat transfer by natural convection is calculated in order to study the effects of the variations in fluid properties. it has been found that the effect of the variations in heat capacity and viscosity is large. The temperature difference and flow rate between the raw-material zone and the crystal-growth zone are also discussed from the viewpoint of the transport phenomena in a crystal-growth process. (c) 2008 Elsevier B.V. All rights reserved.