A method of forming a semiconductor structure is provided. First, a target layer and a mask layer are sequentially formed on a substrate. Thereafter, a first pattern transfer layer having a plurality of openings is formed on the mask layer. Afterwards, a second pattern transfer layer is formed in the openings of the first pattern transfer layer. The mask layer is then patterned, using the first pattern transfer layer and the second pattern transfer layer as a mask, so as to form a patterned mask layer. Further, the target layer is patterned using the patterned mask layer.