A method for forming an isolation layer of a semiconductor device, which comprises the steps of: a) sequentially forming a pad oxide layer and a pad nitride layer on a silicon substrate; b) etching the pad nitride layer, the pad oxide layer, and the silicon substrate, thereby forming a trench; c) thermal-oxidizing the resultant substrate to form a sidewall oxide layer on a surface of the trench; d) nitrifying the sidewall oxide layer through the use of NH3 annealing; e) depositing a liner aluminum nitride layer on an entire surface of the silicon substrate inclusive of the nitrated sidewall oxide layer; f) depositing a buried oxide layer on the liner aluminum nitride layer to fill the trench; g) performing a chemical mechanical polishing process with respect to the buried oxide layer; and h) eliminating the pad nitride layer.