A method of forming a pattern is disclosed. At first, a layout pattern is provided to a computer system. The layout pattern includes at least a first strip pattern and at least a second strip pattern, and a width of the second strip pattern is substantially larger than a width of the first strip pattern. Subsequently, the second strip pattern neighboring the first strip pattern is defined as a selected pattern. Then, an assist pattern is formed in the selected pattern, and the assist pattern does not overlap a center line of the selected pattern. The layout pattern and the assist pattern are further outputted through the computer system onto a mask.