Abstarct: Thin Walled Beams are widely used in engineering applications with the minimum weight design criteria, ranging from civil to aerospace and many other industrial fields. The weight reduction of the beam leads to importance of the dynamic behavior of the structure. Frequently used thin walled beams have low torsional stiffness and their torsional deformations may be of such magnitudes that it is not adequate to treat the angles of cross section rotation as small. When the vibration amplitudes are moderate or large, the geometric nonlinearity must be included and some new phenomena which do not exist in linear torsional dynamic come into play. In this study, free vibration and dynamic stability of thin walled composite beam will be investigated. The governing nonlinear differential equations of motion will be obtained using Hamilton’s principle. A number of specific design effects such as warping, large torsional deformations, periodic axial load and pretwist angle are considered. It is shown that the beam’s torsional deformation has a static equilibrium point. Hence, the governing nonlinear differential equation of motion is linearized near this equilibrium point. The effects of fiber angle, pretwist angle and axial load on the natural frequencies of the system are investigated. Dynamic stability of beam is analyzed using method of multiple scales and instability boundaries of the system are obtained.