A design method of a photomask structure including the following steps is provided. A layout pattern is provided. The layout pattern includes first to third basic patterns. The second basic pattern is located between the first and third basic patterns and connected to the first and third basic patterns. There is a first jog portion between the first and second basic patterns, there is a second jog portion between the second and third basic patterns, and the first and second jog portions are located at two opposite sides of the layout pattern. The first and second jog portions are moved to align the first and second jog portions with each other and to eliminate the second basic pattern, wherein a first area change amount produced by moving the first jog portion is equal to a second area change amount produced by moving the second jog portion.