Abstract: Elaborated in the paper are the R & D activities of the straight welded linepipe as to be used for the deep ocean gas pipeline project. The chemical composition and manufacturing process of the steel for the said linepipe are optimized. And based on the optimization result, the welding process and forming processfor the pipes are conducted. And thanks to using of the four wire mono-layer welding process for pipe inner and outer welding with MK65HGX-ⅢSAW welding wire as developed by the company himself and the SJ102G hi-basicity weld agent, 52, 000 tons of Φ558.8 mm(22 in) SMYS 450F/X65MO SAWL straight welded linepipes for deep ocean gas pipelines is successfully produced. Relevant tests as conducted to the linepipes show that they are all in compliance with DNV-OS-F101:2007 and the customer specified requirements in terms of physicochemical properties, geometric dimensions and appearance quality, etc.