An exemplary method of fabricating a semiconductor device is provided. First and second hard mask patterns adjacent to each other are formed on a substrate. First and second active fins are formed by etching the substrate using the first and second hard mask patterns as a etch mask. An isolation layer is formed to fill a region defined by the first and second active fins and the first and second hard mask patterns. A mask pattern is formed to be positioned on the first hard mask pattern and overlap the first active fin. A first trench is formed by etching a portion of the isolation layer and a portion of the second active fin using the mask pattern as an etch mask. The remaining portion of the second active fin is removed by performing an isotropic etching process.