Abstract: The microstructure and mechanical properties of pipeline steel X80 were investigated by means of optical electron microscope, scanning electron microscope, transmission electron microscope, etc. The experimental results showed that the yield strength of pipeline steel X80 with 16mm wall thickness can be up to over 670 MPa through controlled rolling/cooling, and the ratio of tensile strength to yield strength and ductile-brittle transition temperature are lower than 0.85 and-60°C, respectively, thus providing a nice match between strength and toughness. Fine acicular ferrite has good effect on both the strength and toughness of the steel. Two kinds of precipitates were observed in the steel, where the coarse ones are mainly the Nb and Ti(CN) ranging from 50 nm to 200 nm, and the fine ones are mainly NbC less than 30 nm. Both the nano-precipitates play an important role in strengthening and grain refinement of the steel.