A geometrically nonlinear model for thin-walled, single-cell composite beams is developed by using variational formulation and the variational-asympotical method. The structural modeling is split into two parts: a two-dimensional analysis over the cross section, and a geometrically nonlinear analysis of a beam along the beam span. The nonlinear model is based on the assumption of moderate beam deflection, accounting for the pitch angle and extends the linear analysis model for anisotropic thin-walled beams. By employing the Galerkin’s method, an nonlinear algebraic equations is derived and then solved by means of an incremental Newton-Raphson method. Numerical results are obtained for one cantilevered box beam: Circumferentially Uniform Stiffness(CUS), under external load to investigate the effect of geometric nonlinearity and the effects of the fiber orientation, laminate stacking sequence, are also addressed.