Design of reinforced concrete thin walled open sections subjected to combined loading such as axial force, biaxial bending moment and torsional moment is cumbersome. This is due to the fact that such structures possess very little torsional rigidity and the plane cross section does not remain plane after the deformation. In this study, an algorithm is developed for the optimum design of reinforced concrete thin walled sections. Single core lipped channel shape shear walls are considered as a design example. The design algorithm treats the area of reinforcement bars as variables in addition to cross sectional dimensions of the thin walled section. The limit state theory is used in the formulation of the design problem. In addition to displacement constraints the strain limitations in concrete and the yielding constraints for rebars are considered. A numerical procedure is presented for computations of sectional and sectorial properties. The eect of warping due to torsional deformation of the core is included using Vlasov theorems. The solution of the design problem is obtained employing the optimally criteria method. Application of the algorithm is demonstrated in designing a core structure with a lipped channel cross section.