A process for forming a narrow field oxide layer with a greater thickness. A silicon substrate is provided on which a layer of pad oxide and a layer of nitride are formed. Then, at least a wide area and a narrow area are defined on the silicon substrate through openings on the nitride layer. A thermal oxidation process is performed so as to grow a first oxide layer on the wide area and a second oxide layer on the narrow area. A polysilicon layer is then deposited over the entire surface. After that, chemical-mechanical polish (CMP) is applied so as to rub away part of the polysilicon layer that is lying above a plane coincident with the topmost surface of the nitride layer, thereby leaving a first polysilicon layer on the first oxide layer and a second polysilicon layer on the second oxide layer. A thermal oxidation process is performed so as to oxidize the first polysilicon layer and the second polysilicon layer, thereby increasing the thickness of the first oxide layer and the second oxide layer. The resultant thickness of the oxide layer formed on the narrow area is approximately equal to that of the oxide layer formed on the wide area.