We studied the mechanism of formation of Si3N4 and Si2N2O during the solidification process of multicrystalline silicon by numerical analysis with a phase diagram of the Si(l)-N-O system. Concentrations of oxygen and nitrogen were almost the same as reported values of measurements. Si3N4 was formed on the top of the silicon ingot. Si3N4 was also formed at the middle stage of the solidification process at the center of the ingot. It was clarified from the results that Si2N2O Was first formed near the melt-crystal interface, since oxygen concentration in the melt decreases and nitrogen concentration in the melt increases with solidification of the molten silicon. Si3N4 was formed after Si2N2O had been formed. (C) 2009 Elsevier B.V. All rights reserved.