Provided is a trench filling method, which includes: forming a silicon oxide liner on a semiconductor substrate with trenches formed therein, the trenches including narrow-width portions having a first minimum isolation width and wide-width portions having a second minimum isolation width being wider than the first minimum isolation width; forming an oxidation-barrier film on the silicon oxide liner; forming a silicon liner on the oxidation-barrier film; filling the narrow-width portions with a first filling material; filling the wide-width portions with a second filling material; and oxidizing the silicon liner.