disclosed is a method for fabricating a semiconductor device capable of preventing a depth of a plurality of moats m from getting deeper as preventing lowering a threshold voltage by forming a round shape of a top corner of a trench . particularly , the method includes the steps of : forming a pad pattern by sequentially stacking a pad oxide layer and a pad nitride layer on a substrate ; forming a trench by etching process to an exposed surface of the substrate by using the pad pattern as a mask ; filling an insulation layer for isolating device elements filled into the trench ; removing the pad nitride layer ; performing a pre - cleaning process for removing the pad oxide layer ; selectively recessing the surface of the substrate to remove a plurality of moats m taken place after removing the pad oxide layer ; and forming a screen oxide layer on the surface of the substrate .