Various design loads used in the strength evaluation of ship structures have been introduced currently by classification societies. As most of such design loads have been determined as the standard loads for the sake of convenience, the relationships between the design loads and the sea states actually encountered by ships seem to be poor. Accordingly, it may be difficult to refer and utilize them as the fundamental design concepts for ship designers or the operational guidelines for ship operators. Under these circumstances, authors propose the practical estimation method of the design loads having transparent and consistent backgrounds to the actual loads acting on primary structural members of tankers through the 1st report and 2nd report. In the 1st report, the design sea states that closely resemble the actual sea states, which are considered as the most severe for hull structures are proposed. Furthermore, the practical estimation methods of the design sea states are proposed by parametric studies using the results of series calculation of representative tankers.