Abstract: For the material and geometric nonlinearity of RC columns, a graphic method is derived to calculate the ultimate loads in consideration of the second-order effects and the elastic-plastic performance on the sinusoid deformation assumption. By analyzing the tangent characteristic of the sectional moment-curvature relationship curves, the slenderness ratio and the first-order eccentricity are taken as unknown variables in the method, while the moment-curvature curves are not simplified, and every point on the curve is taken as the ultimate loads directly. The upwards and downwards parts of load-deflection curves are obtained simultaneously by the graded-loading method. The results are in good agreement with the experimental results. The results show that the material strength damage may also occur in slender columns, and its second-order ultimate bearing capacity is approximately equal to the sectional ultimate bearing capacity.