Abstract: Briefed in the paper is the development of the JCOE straight SAW pipe by Julong Steel Tube Co., Ltd. with the X90 and X100 steel plates as developed by Baoshan I & S Corp., Ltd., covering such activities as studying of the mechanical property variation, the weldability and the softening of the heat-affected area of the X90 and X100 steel plates, the comparative tensile test analysis between the result of the test made with a recta- ngular test piece and that of the test made with a round bar test piece, and study on influences on performance of the weld joint by different welding materials. Moreover comparative analysis is made between the X90 and X100 steel plates and the tubes made therewith. It is proposed that round bar test piece be used for tensile tests of the X90 and X100 steel linepipes, and that criteria concerning the test sections of linepipe-purpose steel plate and steel linepipe be established.