After a trench is formed into a substrate, a polysilicon layer is formed on sidewalls and a bottom of the trench. A thermal oxidation is performed on the polysilicon layer such that a polysilicon oxide layer is formed thereon. Then, a portion of the polysilicon oxide layer is removed such that the polysilicon layer is exposed on the bottom of the trench while the sidewalls of the trench are still covered by the polysilicon oxide layer. A TEOS-ozone oxide layer is deposited on the substrate to fill the trench. Since the bottom of the trench has a better condition for the deposition of TEOS-ozone oxide layer than that of the sidewalls, a gap fill quality can be enhanced.