In this paper, a stress-strain relationship in concrete which takes account of the confining effect on the filled concrete is proposed to evaluate ultimate strength and deformation capacity of concrete filled circular steel tubular columns rationally. The confining effect is quantitatively derived from the characterization between Poisson's ratio and axial strain in steel and concrete filled tube under concentric loading. The applicability of the proposed stress-strain relationship in concrete as well as the influence of strain hardening in steel is discussed. Ultimate strength, curvature and deflection of the concrete filled circular columns are calculated on the basis of the proposed model in steel and concrete, comparing with the past test results. It is concluded that the predicted relations agree quite well with the test results.