Abstract: The effects of relaxation time after rolling on microstructure and mechanical properties of X100 pipeline steel by tensile testing,impacting testing and microstructure observing.The results show that with the prolongation of relaxation time,the bainite ferrite laths became wide and dislocation density inside laths reduced,the volume of carbonitride precipitation increased,the yield strength firstly decreased and then increased,the tensile strength increased slowly,the yield ratio and elongation after fracture decreased firstly,then increased and then decreased,and the impact energy at -20℃ increased firstly and then decreased.When the relaxation time was 52 s,higher strength,higher toughness and uniform elongation were achieved,those could meet the demands of large deformation of X100 pipeline steel.