A method for modifying a photomask layout includes the following steps. First, a photomask layout having at least an edge is provided. A plurality of evaluation points are positioned on the edge. Then, the photomask layout is interpreted to have an interpreted photomask layout and an interpreted edge pattern. The interpreted edge pattern is formed by interpreting the above-mentioned edge. After that, a shift between the edge and the interpreted edge and corresponding to each of the evaluation points is calculated. Afterwards, a shift gradient between two evaluation points can be derived from the shift. Finally, a number of segments between each two evaluation points can be estimated.