The effective breadth of a stiffened plate under bending was investigated. The design parameter of an effective breadth was used when a stiffened plate subjected to a lateral loading was considered. Due to the lateral loads there was shear transmission through the plate from the stiffener, which leads to a non-uniform longitudinal stress distribution across the plate width the effective breadth that takes into account a shear lag can be calculated by the simple beam theory. For a tanker, the existing effective breadth concept has been used when longitudinal stiffeners are fixed at adjacent web frames. However, the effective breadth of each classification’s formula can’t be applied to flexible support condition such as the one prevailing in Ro-Ro vessel. The effective breadth according to the rules of several classification societies was compared and a way to get an accurate analysis as suggested.