A method of fabricating a semiconductor device is described. A substrate having a memory cell region and a high voltage circuit region are provided. First and second source/drain regions are formed in the substrate within these two regions. A silicon oxide layer, a first conductive layer and a top layer are sequentially formed over the substrate. A floating gate is defined in the memory cell region and the top layer and the first conductive layer of the high voltage circuit region are removed. The exposed silicon oxide layer is thickened. Thereafter, the top layer is removed and then a barrier layer is formed on the exposed surface of the floating gate. A second conductor layer is formed over the substrate, and then a gate is defined in the high voltage circuit region and a control gate is defined in the memory cell region.