Abstract: Microstructure of high strain line pipe steel was observed and analyzed by optical microscopy and transmission electron microscopy. The results show that the microstructure of the pipe steel is a mixture of soft-phases and hard-phases. The soft-phases are mainly composed of polygonal ferrite and quasi-polygonal ferrite, and the hard-phases are mainly composed of granular bainite, bainite ferrite and lath martensite. The microstructure is uniformly distributed along the wall-thickness direction. The polygonal ferrite grains reach full growth, and most of the polygonal ferrite grain boundaries belong to large angle grain boundaries. The bainite and martensite are between polygonal ferrites with long strip shape, and the phase difference between laths is normally more than 15°.