A method for forming fin structure includes following steps. A substrate is provided. A first mandrel and a plurality of second mandrels are formed on the substrate simultaneously. A plurality of spacers are respectively formed on sidewalls of the first mandrel and the second mandrels and followed by removing the first mandrel and the second mandrels to form a first spacer pattern and a plurality of second spacer patterns. Then the substrate is etched to simultaneously form at least a first fin and a plurality of second fins on the substrate with the first spacer pattern and the second spacer patterns serving as an etching mask. At least one of the second fins is immediately next to the first fin, and a fin width of the first fin is larger than a fin width of the second fins. Then, the second fins are removed from the substrate.