It is important to estimate the ultimate longitudinal strength of a ship hull girder, because it is the real resistant value to the wave bending moment. The ultimate bending moment of its cross-section was calculated by the in-plane strength of stiffened plates. The author's 3 rd report presented two approximate formulas to estimate the ultimate strength of the typical stiffened plates. These formulas took into account the influence of initial imperfection. In this report, a simple method using these formulas is proposed for an estimation of the ultimate bending moment. Experiments were carried out by using nine box girder models under pure bending loading. The calculated ultimate bending moment agrees well with the experimental results and have confirmed efficiency in first design stage. The ultimate bending moments of the cross-section at the midship are calculated by this method and the characters are studied. It is found that the ultimate bending moment is strongly relative to the ultimate strength of the deck plate or bottom plate under compressive loading.