A method for forming a shallow trench isolation is disclosed. The method avoids using any silicon nitride material to prevent the kooi effect and use spacers to protect the corner portions of the STI. A conductive layer is used to replace the conventional used silicon nitride layer in the formation of conventional STI regions. The invention also uses a dielectric layer comprising a pad oxide layer as a sacrificial oxide layer so that an additional sacrificial oxide layer is no longer needed. The conductive layer will be oxidized together with the substrate in the formation of the gate oxide layer so that the isolation quality will not be degraded.