Si/SiO2interface structures in laser‐recrystallized Si on SiO2were studied with a high‐resolution transmission electron microscope. The (001) Si/SiO2interface with (001) Si substrate as a seed was excellent in flatness, flatter than that of the initial interface before recrystallization. However, the (11¯5) Si/SiO2interface with (11¯5) Si substrate was saw‐toothed with {100}Tand {111}Tmicrofacets. After twin boundary generation, the interface was changed to {110}Tor {111}Tand was flattened considerably. A Si/SiO2interface reaction occurred during laser recrystallization. Since low‐index Si planes are thought to have low interface energies with SiO2at their interface, atomically flat or saw‐toothed interfaces appeared as a result of this interface reaction. Moreover, twin boundaries, rather than saw‐toothed interfaces, might have been generated for the reduction of the interface energy.