Abstract: The transverse internal force in the flanged top panel of single-box and single-cell reinforced concrete box girders after concrete cracking is discussed, and the nonlinear behavior of the effective distribution width in the panel is presented. The effective distribution widths in the flanges under two different boundary conditions are also analyzed. The numerical result shows that, in contrast to the case of linear-elastic solution, there existed a significant difference when the material non-linearity effect and boundary condition were considered. The numerical result also shows that, based on the stress level of the concrete and reinforcement, it is rational to increase the effective width obtained from the present bridge code JTG D62 up to 40 percent for the design of the top panel, while in another case, because of the rigid restriction of the diaphragm, it is reasonable to decrease the effective width of the present bridge code to an extent of 50 percent for the design of flange near the diaphragm. Based on the parametric study, the elastic-plastic effective distribution width concept is proposed, and a revised method of evaluating box panel's effective transverse width considering concrete cracking is presented and compared with those bridge codes at home and abroad.