There is growing interest and investment in the renewable energy industry with the goal of promoting eco-friendly growth globally. The proportion of wind power generation systems in the world’s renewable energy market is kept continuously increased and is known as 21.9%. In order to reduce the fossil-fuel power generation capacity, many countries enlarge the investment of the wind power generation systems. As the size of blade enlarges, the installation cost decreases and then the power generation efficiency could be increased. However, as the sizes of the blade and tower are enlarged, the weight and the wind load of itself might be increased. Therefore, it is necessary to develop appropriate components and their build-up technology that can guarantee the structural safety of the wind turbine. In this study, the stress applied to the tower flange when the wind turbine is operating is numerically analyzed. In particular, optimal design value for the aspect ratio of the tower flange was obtained, using fluid-structure interaction (FSI) method. The lowest von Mises stress had an aspect ratio of h1/h2 = 1.40. Numerical analysis of local stress of the tower and flange was calculated using the commercial code ANSYS 18.1.