Oxygen impurities can reduce the carrier lifetime of mc-Si solar cells. In this study, simulations of the transient temperature, velocity and concentrations of oxygen and silicon oxide are carried out in order to clarify the transport mechanism of oxygen impurities in the silicon melt and silicon oxide through argon gas, in a directional solidification system (DSS) furnace. As the solidification fraction enlarges, the oxygen concentration in the melt diminishes, because of the reduction in the amount of crucible surface immersed below the silicon melt. When the solidification fraction is small, two pairs of vortices appear in the melt. Oxygen originating from the crucible is carried towards the free surface by the upper vortex. Oxygen concentration is higher with a higher furnace pressure rather than with a lower one due to the low SiO evaporation at the free surface. When the solidification fraction increases, the upper vortex gradually disappears. The lower vortex occupies almost the whole of the melt, with the exception of a small upper central region where a small vortex forms because of the cooling effect of the argon gas. Oxygen impurities carried by the lower vortex along the crystallization front towards the central region are obstructed by this small vortex. The size of the small vortex increases as the solidification fraction increases. Since the small vortex is stronger when the furnace pressure is higher, the concentration is lower around the central region. This means that the oxygen concentration is smaller when the furnace pressure is higher rather than lower. The simulation results agree well with the experimental results. (C) 2010 Elsevier B.V. All rights reserved.