We investigated the grain growth behaviors of polycrystalline silicon during directional growth from melt. Two types of grain growth behaviors were directly observed using an in situ monitoring system. In the first, when the moving velocity of solid-liquid growth interface is slow and the interfacial morphology is flat, a grain with a plane of lower surface energy with respect to the growth direction expands to lateral direction. In the second, when the interface moves fast and it has an irregular shape because of the differences of the growth rate among grains, a faster growing grain competitively expands to lateral direction covering the slower one. We suggested that the undercooling at the growth front is the key parameter to divide those growth behaviors. (C) 2004 Elsevier B.V. All rights reserved.