Abstract: The microstructure of X100 pipeline steel produced by steckel mill was observed by SEM and TEM. Electron backscattered diffraction (EBSD) was employed to investigate the relationship between effective grain size and low-temperature toughness. The size distribution and strengthening effect of precipitation in X100 pipeline steel were quantitatively analyzed through physicochemical phase analysis technology. The results show that the microstructure of X100 pipeline steel mainly consists of granular bainite with plenty of M/A islands dispersing in the matrix or on grain boundaries. The effective grain size of X100 pipeline steel is refined to about 2μm. The refinement of effective grains and the reduction of microstructure orientation are two effective ways to improve the low-temperature toughness. The average size of precipitation particles in X100 pipeline steel is refined to 45.4 nm. Since the total mass fraction of precipitation particles is about 0. 062%, the strengthen effect of precipitation is only about 52 MPa, which does not contribute much to the yield strength.