The ultimate strength of concrete filled rectangular steel tubes under the combined actions of axial compressive forces and bending moments, so-referred to as the composite beam-columns, is analyzed on the basis of computer program by using CDC method. In this method, both the initial imperfections such as residual stress and initial deflection of steel tubular plates and the non-linear behaviors of steel plates and encased concrete materials are exactly taken into the considerations. Through the various numerical calculations, the properties and variations of ultimate strength for the composite beamcolumns due to initial imperfections, cross-sectional configurations, slenderness parameters, load contribution factors between steel and concrete as well as applied load ratio are inquired. Based upon these parametric analyses, the approximate and practical interaction curves between applied axial compressive force and bending moment are investigated and their validities are checked in comparison with the design codes of DIN 18806 and BS 5400 Part 5.