Abstract: An improved isolation method in a semiconductor device of selective polysilicon oxidation (SEPOX) which can create a field oxide layer having a size below the optical resolution and good isolation characteristics. A buffer layer comprised of polysilicon or amorphous silicon is formed on a semiconductor substrate, and then an anti-oxidative pattern with an opening which defines an isolation region exposing a portion of the buffer layer is formed. Then a portion of the exposed buffer layer is isotropically etched in order to form an undercut portion in the lower portion around the opening. Then an anti-oxidative spacer filling the undercut portion is formed on the sidewall of the opening. Thereafter, a field oxide layer is formed by partially oxidizing the portion of the buffer layer exposed by the opening and the semiconductor substrate exposed in the opening. The size of bird's beak is decreased, thereby forming a field oxide layer with good isolation characteristics and small size.