A flap acts as a control surface or a high-lift device. The prediction of its aerodynamic performance is important in the development of aircraft and it is usually conducted with wind tunnel tests. To investigate the usefulness of numerical methods, the aerodynamic characteristics of a low aspect ratio wing are analyzed with numerical methods. The wing has an unswept untapered planform and a plane flap near the trailing edge. The wing configuration and experimental data are from the NACA report. The considered numerical methods include panel methods and computational fluid dynamics. The results of aerodynamic analysis of each numerical method are compared with experimental data.