The growth behavior of faceted Si crystals in grain boundary formation during crystallization was studied by the in situ observation technique. We directly observed the transition of the shape of the growing interface just before the impingement of two crystals. It is found that when a crystal with a zigzag-shaped faceted interface encounters another crystal, {111} facet facing on the growing interface gradually disappears with the transition of the interfacial shape from zigzag to linear. This shape transition of the growing interface determines the grain boundary shape and grain boundary character. (C) 2009 Elsevier B.V. All rights reserved.