Most of the structural elements in a steel structure are subjected to bending moment and axial force simultaneously. In some elements, one of the two components is relatively small compared to the other. Yet, the smaller component cannot be ignored due to the interactive behavior of the two components. Therefore, it is not adequate to design the beam-column element as a beam or a column by ignoring one of the two load components even if the ignored component is relatively small.Most of the design codes use empirical interaction equations that are based on semi-experimental semi-analytical results. Most of the design formulae adopted by the design codes do not explicitly account for the geometrical imperfection.This research aims at investigating the buckling behavior of steel beam-column elements for the sake of developing an analytical model to calculate their ultimate resistance under axial compression and bending moment. The analytical model will be based on Perry-type formulation, and it will account for the effect of initial imperfection. The model will be validated by comparing its results with those obtained by the Finite Element Non-Linear Elasto-Plastic analysis using ANSYS 5.4 program.Finally, a simple but rational design method based on the model, will be introduced. This method can be applied using a simple mathematical expression or charts and tables. The results of the developed design method will be compared with the design method of the international codes of practice for design of steel structures. On light of these comparisons, design recommendations are introduced