A method for forming an isolation layer of a semiconductor device includes forming a trench in a substrate, forming a high-density plasma (HDP) oxide layer filling a portion of the trench, forming a spin-on-dielectric (SOD) oxide layer having a certain height over the HDP oxide layer, performing a thermal treatment, and forming an enhanced high-aspect-ratio process (eHARP) oxide layer filling another portion of the trench over the SOD oxide layer.