A method for fabricating an STI gap fill oxide layer in a semiconductor device is provided. The method can include: forming a shallow trench for forming an STI on a semiconductor substrate; forming an STI liner oxide layer in the shallow trench for the STI; depositing an APCVD oxide layer at an upper portion of the STI liner oxide layer for an oxide layer gap fill in the shallow trench of the STI; d) performing a densifying annealing process to densify the APCVD oxide layer; and depositing an HDP-CVD oxide layer at an upper portion of the APCVD oxide layer so that the STI shallow trench is completely gap-filled.