A method of forming a semiconductor fin structure is provided. A substrate is provided, which has at least two sub regions and a dummy region disposed between the two sub regions. A recess is disposed in each sub region. A semiconductor layer is formed to fill the recesses. A patterned mask layer is formed on the semiconductor layer in the sub regions and on the substrate in the dummy region. The substrate and the semiconductor layer are removed by using the patterned mask layer as a mask, thereby forming a plurality of fin structures in the sub regions and a plurality of dummy fin structures in the dummy region.