Abstract: The microstructures and mechanical properties of three X80 pipeline steel plates were investigated in this study. The plates were hot rolled by two stages, and cooled by three different cooling schedules to obtain different microstructures and mechanical properties. Transmission electron microscope (TEM) was used to observe the microstructural details in the steel. The results indicate that the amount of polygonal ferrite (PF) and quasi-polygonal ferrite (QPF) decrease with decrease of finish cooling temperature (FCT), while the amount and size of M-A islands are larger. Relaxation promotes the Charpy V notch(CVN) property at -20℃, but not the drop weight tear testing(DWTT) property at -15℃, and decreases the strength of the steel.