A pattern decomposition method including following steps is provided. A target pattern is provided, wherein the target pattern includes first patterns and second patterns alternately arranged, and the width of the second pattern is greater than the width of the first pattern. Each of the second patterns is decomposed into a third pattern and a fourth pattern, wherein the third pattern and the fourth pattern have an overlapping portion, and a pattern formed by overlapping the third pattern and the fourth pattern is the same as the second pattern. The third patterns and the first pattern adjacent to the fourth pattern are designated as first photomask patterns of a first photomask. The fourth patterns and the first pattern adjacent to the third pattern are designated as second photomask patterns of a second photomask.