An isolation method in a semiconductor device is disclosed. The example method sequentially forms a pad oxide layer and a pad nitride layer on a semiconductor substrate, patterns the pad nitride and oxide layers to form an opening exposing a portion of the substrate, and forms a trench in exposed portion of the substrate. The example method also etches the patterned pad nitride layer to extend the opening, carries out SAC oxidation on the extended opening and the trench to provide a rounded corner to an upper corner of the substrate in the vicinity of the trench, and fills the trench with an insulating layer.