Abstract: Before studying the local buckling of webs of composite beams subjected to positive bending moment, the buckling interaction formulas of a simply supported rectangular plate subjected to combinations of tension, bending and shear stresses should be established. The interaction formulas of elastic buckling of such a plate are obtained through the energy method. The results are in an excellent agreement with those obtained from commercial finite element software ANSYS. A simple formula is then presented. Based on the interaction formula, the elastic limiting height-to-thickness ratio is given, which is also compared with the ratio without considering bending and tension. It is demonstrated that the height-to-thickness ratio obtained is increased, and in most cases, it is more than 30% larger. Results obtained provide a good foundation for the research of elasto-plastic buckling of webs of composite beams considering residual stress and initial imperfection.