The melting and solidifying behaviors of a thin Si film have been studied. The morphological imperfections can be inhibited by a higher thermal gradient at the melting front and the density of constitutional defects （as grainboundaries and dislocations）can be decreased by a lower thermal gradient at the solidifying front.A novel structure of graphite heater has been invented to meet the different demands on radiative heating. Successful fabrication of high quality SOI has been carried out.