Abstract: A method of fabricating a shallow trench isolation (STI) structure is provided. A pad oxide layer, a pad silicon layer and a mask layer are sequentially formed over a substrate. Thereafter, the mask layer and the pad silicon layer are patterned to form an opening that exposes a portion of the pad silicon layer. A spacer is formed on the sidewall of the opening and then a portion of the pad oxide layer is removed to expose a portion of the substrate. A portion of the substrate is removed to form a trench. The spacer is removed and the pad oxide layer is etched back to form an undercut. Next, insulating material is deposited over the substrate to fill the trench. The patterned mask layer, the pad silicon layer, the pad oxide layer and a portion of the insulating material are removed to form the STI structure.