Abstract: With Gleeble 3500 thermal simulator,the transformation behavior of the acicular ferrite of the Nb microalloyed pipeline-purposed steel during continuously cooling process is studied. The specimen is heated up to 1 050 ℃to get austenitized,and then cooled down to 850 ℃,and isothermally held on for 0 s,300 s,600 s and 1 200 s,and then cooled to the room temperature.The Jeziorny kinetics formula of the acicular ferrite of the Nb micro-alloyed steel is established through phase transition volume-temperature curve as calculated by the lever law according to the thermal expansion curve.The results indicate that along with increase of the thermal holding duration,precipitation of the Nb(C,N)particle is promoted,which provides nucleation sites for acicular ferrite transformation,so as the phase transformation temperature is increased,and accordingly the acicular ferrite transformation is promoted,whereas the bainite transformation is restrained;and finally the bainite-acicular ferrite mixed microstructure is transformed into the pure acicular ferrite microstructure;and the phase transformation starting time is reduced,and the phase transformation ending time is accordingly shortened,while the volume fraction of the acicular ferrite increases.