Using secondary‐ion mass spectroscopy, we have found that oxygen is strongly concentrated at the sub‐boundaries in zone‐melting‐recrystallized silicon‐on‐insulator films prepared by the graphite strip heater technique. This observation suggests that the formation of sub‐boundaries during recrystallization may be caused by constitutional supercooling resulting from the presence of oxygen that is dissolved into the molten Si zone from the adjacent SiO2layers.