Abstract: This paper proposes a novel method for determining the equivalent moment factor for checking the in-plane stability of beam-columns. The proposed method uses the total second-order bending moment caused by initial deflection and the linearly varied bending moment. By introducing the total bending moment at each position along the member length into the cross-sectional strength formula， a series of axial force-bending moment interaction curves are obtained. The lower-bound envelope of these curves constitutes the final interaction curve for the in-plane stability check. Based on the obtained interaction envelopes， the elastic-plastic equivalent moment factor is derived inversely， and approximate formulas are proposed. Comparisons show that the proposed formulas are in good agreement with the results from the proposed method.