A strain-based design has been widely discussed in recent years, particularly for the use of high strength steel line pipes. Line pipes for strain-based design applications are required to have not only good toughness and weldability but also high deformability such as low yield to tensile strength (Y/T) ratio, high work hardening rate (nvalue) and high uniform elongation (U.El). We studied how to simultaneously improve toughness and deformability in high strength steel line pipes through laboratory research. In addition, we considered the means to control change in the mechanical properties by thermal aging during anti-corrosion coating treatment. Basic study in a laboratory indicated that a dual phase microstructure, which consists of bainite and fine polygonal ferrite, was effective for simultaneously improving deformability and toughness instead of bainite single microstructure. We confirmed the optimized volume fraction and grain size of polygonal ferrite to achieve excellent elongation, Y/T ratio and DWTT property to line pipes for a strainbased design application. Moreover, the basic study revealed that the accelerated cooling condition in hot rolling absolutely correlated with thermal aging during the coating treatment and that reduction in the solute carbon content in steel was the most effective way to suppress strain aging. Based on the experimental results, we experimentally manufactured X80 UOE pipes for strain-based design applications, which had a low Y/T ratio, high uniform elongation, good Charpy impact and DWTT properties, in a commercial mill.