A method for forming shallow trench isolation is disclosed. The method includes forming a trench in a semiconductor substrate, and then blanket depositing a silicon oxide layer over the semiconductor substrate by a plasma process, thereby substantially refilling the trench. Thereafter, a photoresist layer is formed on the plasma deposited silicon oxide layer, followed by etching back a portion of the photoresist layer. The plasma deposited silicon oxide layer is then isotropically etched, and the photoresist layer is then finally removed.