The concept of external post-tensioning has been ussed as an effective technology to strengthen composite steel-concrete girders in bridges. External post-tensioning method, which is practical and economical, has been applied in concrete bridges. The effect of composite bridges strengthened with external post-tensioning has being studying, but it has been applied to a few existing bridges. Therefore, a specific presentation needs for efficient strengthening method related to determination of resonable initial tendon force and a number of tendons. This study shows the process of determination of initial tendon force and a number of tendons using proposed rating factor equation for efficient tendon configuration to improve of load-carrying capacity in a two-span continuous composite girder. This method has been shown as an effective one for an existing two-span continuous composite girder.