A new method of forming device isolation regions on a silicon substrate is provided. This method comprises the following steps: a pad oxide layer is formed on the silicon substrate; a silicon nitride layer is formed on the pad oxide layer; portions of the silicon nitride and pad oxide layers not covered by a mask pattern are etched through and into the silicon substrate so as to provide a plurality of wide and narrow trenches within the silicon substrate that will form the device isolation regions; silicon nitride spacers are formed on the sidewalls of the trenches; a first field oxide layer is grown on bottoms of the trenches by using thermal oxidation wherein a thin oxide layer is also formed on the silicon nitride layer; the thin oxide layer, silicon nitride layer, silicon nitride spacers and pad oxide layer are removed, respectively; and a second field oxide layer is formed on the first field oxide layer by using liquid phase deposition so as to fill all of the trenches.