Eigenshape based imperfection method for beam-columns with mono-symmetric I-sections. This paper shows for the first time the application of the Overall Imperfection Method (OIM) for columns, beams and beam-columns with mono-symmetric I cross-section. The buckling failure mode of compressed members with mono-symmetric I-section is torsional-flexural like. The second-order effect from the rotation around the longitudinal shear axis during the loss of stability is taken into account when the amplitude of the initial imperfection is calculated for the compressed members. For beams, the buckling behaviour is also different from doubly symmetric cross-section because the shear centre and the centre of gravity of the cross-section do not meet at the same position. This paper shows the extension and the usability of the OIM for steel structures with mono-symmetric I-section. The automatic determination of the equivalent amplitudes for columns, beams and beam-columns are presented for the first time. The practicality of the proposed method is demonstrated for single and multi-span members with three examples.