Abstract: Current available theories for thin-walled curved beams lack a rigorous theoretical development.This paper provides a detailed derivation of an exact theory for biaxial bending and torsion of thin-walled circularly-curved beams .The derivation is based on two well-accepted assumptions in the theory of straight thin-walled beams. Exact expressions for warping displacement, normal and shear stresses and their resultants are presented. A simplifed theory is also given for practical applications.