A new process involving ultra-fast cooling (UFC) and on-line tempering (OLT) was proposed to dis-place austempering process, which usually implements in a salt/lead bath and brings out serious pol-lution in the industrial application.The optimization of the new process, involving the evolution of the microstructure of medium-carbon steel during various cooling paths, was studied.The results show that the cooling path affected the final microstructure in terms of the fraction of pearlite, grain size and distribution of cementite in pearlite.Increasing the cooling rate or decreasing the OLT tem-perature contributes to restraining the transformation from austenite to ferrite, and simultaneously retains more austenite for the transformation of pearlite.It is also noted that bainite was observed in the microstructure at the cooling rate of 45°C/s and the OLT temperature of 500°C.Through either increasing the cooling rate or decreasing the OLT temperature, the distribution of cementite in pearli-te is more dispersed and grain is refined.Taking the possibility of industrial applications into account, the optimal process of cooling at 45°C/s followed by OLT at 600°C after hot rolling was determined, which achieves a microstructure containing nearly full pearlite with an average grain size of approxi-mately 7μm and a homogeneously dispersed distribution of cementite in pearlite.