The paper considers a plane stability problem of thin-walled bar of angular profile made of an isotropic linearly elastic material. The critical parameters were determined with the aid of the second version of three-dimensional linearized stability theory and the ground state from equations of the linear theory of elasticity. The approximate solution of the stability problem was sought by the net method. The dependence of the critical parameters on the thin-walledness parameter of structure has been examined, and the errors of the applied stability theory, which is used for the design of frameworks, have been determined.