Multicrystalline silicon (mc-Si) produced by unidirectional solidification system is a crucial photovoltaic material due to its relatively high conversion efficiency and low cost. Defects related to thermal stresses, for example, dislocation, significantly affect the performance of the material. In the paper, a global transient model is applied to examine effects of gas flow on stress levels inner the silicon ingot during the cooling process. The maximum von Mises stresses under different inlet gas velocities are presented as a function of the cooling time. Stress level with a high inlet velocity at the initial cooling is slightly lower, but is much larger at the late cooling than that with a slow velocity. An optimized condition with variable gas velocities at the inlet is proposed to improve the quality of silicon ingot by reducing the stress level during the cooling process. (C) 2015 Elsevier Ltd. All rights reserved.