In order to understand the characteristics of surface patterns on silicon melt in Czochralski (Cz) furnaces, we conducted a series of unsteady three-dimensional (3-D) numerical simulations of thermocapillary flow of a shallow molten silicon pool with Cz configuration under microgravity. The crucible sidewall is maintained at constant temperature. Bottom and free surfaces are adiabatic or allow heat transfer in the vertical direction. The simulation results indicate that two flow transitions occur when increasing the radial temperature difference along the free surface for 3-mm-deep layer. At first, the steady two-dimensional (2-D) flow becomes steady 3-D flow and then oscillatory 3-D flow. But the steady 2-D flow exhibits direct transition to an oscillatory 3-D flow for 8-mm-deep layer. The critical conditions for the onset of the instability were determined. Characteristics of the steady and the oscillatory 3-D flows were discussed. (C) 2004 Elsevier B.V. All rights reserved.