The distribution of stress in a nonrectangular compressed area, such as a section under a biaxial moment, is different than that of a rectangular compressed area. The properties of the compressive stress distribution of concrete have only a minor effect on the pure bending moment capacity of reinforced concrete beams, but they are important influencing factors for columns subjected to a combined axial load and a biaxial bending moment. The variation of ultimate strain of concrete according to the angle and depth of a neutral axis was investigated. Thus, the modified rectangular stress block (MRSB) model for nonrectangular compressed areas is formulated and compared to the experimental results and the existing RSB model. The MRSB method is able to provide more accurate predictions of test results for the biaxial bending moment strength than the RSB method.