A semiconductor device and fabricating method thereof are disclosed. The method includes forming a polysilicon layer on a semiconductor substrate including a high-voltage area and a low-voltage area, partially etching the polysilicon layer in the low-voltage area, forming an anti-reflective layer on the polysilicon layer to reduce a step difference between the high-voltage and low-voltage areas, forming a photoresist pattern in the high-voltage and low-voltage areas, and forming a high-voltage gate and a low-voltage gate by etching the polysilicon layer using the photoresist pattern as an etch mask.