A method for filling silicon oxide materials into a trench includes providing a substrate having a plurality of trenches, performing a first deposition process to form a first silicon oxide layer in the trenches, and performing a second deposition process to form a second silicon oxide layer in the trenches. The reactant gas of the first deposition process has a first O.sub.3/TEOS flow ratio larger than a second O.sub.3/TEOS flow ratio of the reactant gas of the second deposition process.