Abstract: A cross frame was selected as a most simple and typical case of the multi-storey and multi-span frames which buckle in the out-plane direction. In this paper, the out-plane buckling behavior of the axial loaded column in the cross frame was investigated theoretically and its design criterion was presented. Firstly, the column in the cross frame could be treated as an equivalent model of a compressed column with a lateral spring support at mid height of the column. The elastic buckling load and the effective slenderness of the column in the equivalent model was obtained by theoretical derivation and finite element analysis. Afterward, the ultimate load-carrying capacity of the column with single and two wave imperfections was obtained by using finite element analysis. It was proved that the ultimate load-carrying capacity of the equivalent model could be predicted by the column curve in the current design code for steel structures (GB50017-2003) directly, depending on the effective slenderness obtained herein. In addition, it was found that the full bracing requirement at mid height of the inelastic column was lower than that of the elastic one, hence a deep analysis was carried out and an equation to determine the inelastic critical stiffness was proposed. Based on this inelastic critical bracing stiffness, a new design formula was conducted for the purpose of a more simplified and accurate application in the design of the bracing column loaded axially.