Abstract: Structral robustness means that a structure has the ability to avoid damages disproportional to the initial local damages when suffering accidental events. From previous research, few quantitative theory of robustness has been proposed. In this paper, the definition of the vulnerable component is proposed. The calculation formula of the vulnerability coefficient of homogeneous rectangular component under axial force, shear and flexure is derived. Similarly, the vulnerability coefficient for rectangular reinforced concrete (RC) members under the combined loadings of axial force, shear and flexure is also obtained. Then, according to the theory of the vulnerability coefficient of component in existing research, a new robustness index used for evaluating the robustness of structure is proposed. Finally, a planar truss and a planar frame are calculated to illustrate the application of the robustness index. From the analysis, it can be concluded that reducing importance coefficinents of components and decreasing vulnerability coefficients of components are two effective ways to enhance structural robustness.