The flanges of steel girders in the compressive region are strengthened with the longitudinal steel stiffeners in general practices. The new details of concrete-filled longitudinal ribs have been proposed to strengthen the compressive flanges in steel girders in this study and its behaviors have been evaluated through both experimental and numerical studies. The concrete-filled longitudinal ribs may contain the special type perfobond rib shear connectors to increase the composite action between the steel flange and concrete part in the rib. The transverse rebars are installed through the U-type dowel spaces in the perfobond ribs to improve the composite action between the concrete part and longitudinal steel rib also. The both ends of transverse rebar are welded to the insides of steel rib. The transverse rebars are found to play important roles for the composite strengths of the proposed details in the section under the bending moment. When the longitudinal ribs are welded to the compressive steel flanges, the thermal-prestressed tensile stresses may be introduced to the flanges that will experience the compressive stress due to the service actions. The longitudinal ribs may be designed with high-strength steel than the flange steel, because the longitudinal ribs will be subjected to the thermal-prestressed compressive stresses.