Abstract: A new kind of ultra-low carbon X120 pipeline steel was prepared in laboratory. Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) were used to observe the microstructure of X120 pipeline steel. The results show that the microstructure of the steel was primarily characterized by lath-bainite and acicular ferrite. The size of the single lath-bainite was 2-5 μm in length and 0.2-0.5 μm in width. A group of bainite laths in the size 2-5 μm was composed of a number of single lath-bainite. Particles of (Nb, Ti)(C, N) and CuxS(x=1, 2) were observed, and there were high density of dislocations in bainite laths. The crystallographic orientation in a whole bainite laths was uniform, but distributed with random in different bainite laths. There were a large number of high angle boundaries and low angle boundaries among the fine bainite laths.