The silicon oxide growth in narrow spacing is affected by the compressive stress present in the oxide and a reduction of the growth rate (field oxide thinning) can occur. In this work it is shown that the stress present in the growing oxide under the oxidation mask induces a field oxide thickening phenomenon which is able to reduce the field oxide thinning one. Furthermore, the comparison between wet and dry oxidation points out the influence of the ammonia in developing the field oxide thinning.