In t.~is study, a method to design hull structures of surface effect ships with minimum weight is proposed, and computer programs following the method are also developed. The proposed method considers structural characteristics inherent to surface effect ships by rational structural analyses, but adopts a simplified analysis model to enhance computing efficiency during optimization process. The proposed method uses design loads and strength criteria suggested by the class rules of DnV, but the interaction effect of longitudinal girders and transverse web frames are considered by the simple and accurate grillage analysis method. As design of the midship section is accomplished through optimum design of partial structures such as stiffened plates and complex girders and frame structures, global optimization of all design variables is avoided and computing efficiency is raised. And also, the proposed method contains the simple torsional strength and analysis routine and optimization process of transverse bulkheads against pitch connection moment. Applying the proposed method to ship design, 20% reduction of hull weight was confirmed, and it can be shown that hull weight varies with the change of frame space and there exists optimum frame space.