A high-resolution transmission electron microscopy study of XeCl2 pulsed laser beam crystallized silicon thin film, deposited onto SiO2 at 520 degrees C by low pressure chemical vapor deposition, was carried out so that the formation of a new hexagonal phase and various types of defect in the new phase could be understood on an atomic level. When an amorphous silicon thin film was crystallized by a pulsed laser beam, the resulting polycrystalline matrix contained crystals that had either a hexagonal or a diamond cubic crystal structure. In the interior of the grain which had a hexagonal crystal structure, dislocations and extrinsic type stacking faults were observed. It was thought that the formation of the new hexagonal phase and various types of defect was probably related to the high temperature gradients at the liquid-solid interfaces or to growth breakdown.