Abstract: A method for forming oxide layer on conductor plug of trench structure is proposed. The invention includes following essential steps: First, provide a substrate where a trench locates inside the substrate, herein the trench is partly filled by a conductor plug. Second, forms a plasma enhanced tetraethyl-orthosilicate layer on the substrate, herein the plasma enhanced tetraethyl-orthosilicate layer also fills the trench and covers the conductor plug. Finally, removes the plasma enhanced tetraethyl-orthosilicate layer until the substrate is not covered by the plasma enhanced tetraethyl-orthosilicate layer, herein the conductor plug still is covered by the plasma enhanced tetraethyl-orthosilicate layer. Additional, advantages of application of plasma enhanced tetraethyl-orthosilicate layer comprise compacted structure and high deposit rate.