Abstract: The phase transformation of X80 pipeline steel at different cooling rates and the influence of cooling rate on the microstructure and hardness were studied by using Gleeble-3800 thermal simulation testing machine, scanning electron microscope, metallographic microscope and hardness tester. Subsequently, the continuous cooling transformation curve of the pipeline steel was plotted by combining thermal expansion method and metallographic-hardness method. The results show that thermal deformation promotes the formation of acicular ferrite within the cooling rate range of 10-20 ℃/s. However, no acicular ferrite zone appears in the static CCT curve. Thermal deformation promotes phase transformation during the cooling process, resulting in higher phase transformation temperature of dynamic CCT than that of static CCT. By contrast, increasing the cooling rate will suppress the phase transformation during the cooling, and the higher the cooling rate, the more severe the phase transformation hysteresis and the lower the phase transformation temperature. Besides, the hardness increases with the increase of the cooling rate due to different microstructures formed at different cooling rates, which is the key factor affecting the hardness rather than the precipitates.