A method of manufacturing a semiconductor device may include forming active patterns, forming a polygonal mask pattern having a first width and a second width on the active patterns, forming an active region by executing a first etching process using the mask pattern, forming a first cutting mask for removing a first corner rounding in which a width of the active region is the first width, removing the first corner rounding by executing a second etching process using the first cutting mask, forming a second cutting mask for removing a second corner rounding in which the width of the active region is changed from the first width to the second width, and executing a third etching process using the second cutting mask.