The governing equations for composite thin-walled beams were derived. The theory presented here is suitable for either open-section or closed-section beams of any shape, laminate stacking sequence, and boundary conditions. Under more general assumptions than those of Vlasov, the equilibrium equations consist of seven ordinary differential equations. Further, these seven equations were reduced to four coupled ordinary differential equations, which govern the shear deformation of the middle surface. In the numerical examples, displacements of channel beams of composite laminates were calculated according to the present beam theory and compared with the finite element results and other existing theories.