A method of forming an overlay mark is provided. A plurality of photoresist patterns are formed on a substrate. Each of the photoresist patterns includes a first strip and a plurality of second strips arranged in parallel. The first strip crosses the second strips to form a fence shape. Further, there is a space between two adjacent photoresist patterns, and the space is fence-shaped. A plurality of islands are formed in each of the spaces to form dot type strip patterns. The photoresist patterns are removed, and the dot type strip patterns serve as the overlay mark.