A method of manufacturing a semiconductor device includes forming a plurality of active fins over a semiconductor substrate, sequentially forming first and second hard mask layers over the active fins, forming a first hard mask pattern by etching the second hard mask layer, trimming the first hard mask pattern to form a trimmed hard mask pattern, forming a first photo resist pattern over the first hard mask layer, forming second hard mask patterns by etching the first hard mask layer by using the trimmed hard mask pattern and the first photo resist pattern as an etching mask, and forming active fin patterns by etching the active fins by using the second hard mask patterns as an etching mask.