Abstract: The microstructure of X100 pipe steel during continuous cooling transformation was studied by thermal simulation testing and optical microscopy, and then the tested steel was hot rolled and the effects of different cooling rates following rolling on microstructure and mechanical properties of the tested steel were studied. The results show that the microstructure changed from polygonal ferrite, granular bainite to bainitic ferrite with the increase of cooling rate. The microstructure comprised by granular bainite and bainitic ferrite and excellent mechanical properties were obtained when the cooling rate was 20-42 ℃·s-1.