A method of forming a trench isolation structure prevents a nitride liner from being over-etched, i.e., prevents the so-called dent phenomenon from occurring. An etching mask pattern is formed on a semiconductor substrate. A trench is formed in the substrate by using the etching mask pattern as an etching mask. A nitride liner, serving as an oxidation barrier layer, is formed at the sides and bottom of the trench, and is then annealed in a furnace to density the same. In a subsequent etching process, such as that used to remove the etching mask pattern, the densified nitride liner resists being etched. Accordingly, a trench isolation structure having a good profile is produced.