The effect of relaxation treatment after finish rolling on microstructure and mechanical properties has been investigated for a vanadium and nitrogen microalloyed low carbon bainitic steel.Finer lath bainite microstructure can be obtained in the plate with relaxation.The results of quantitative statistics show that in the plate without relaxation,80% of the total bainite lath bundles are in the range 5-15μm in length and 3-13μm in width,while in the plate with relaxation 80% of the total bundles are in the range 3-9μm in length and 1-7μm in width.The mechanical properties show that the plate with relaxation has higher impact energy,yield strength and hardness than the plate without relaxation,also the comprehensive performance after tempered at 650℃ is superior to the plate without relaxation. The effect of relaxation treatment after finish rolling on microstructure and mechanical properties has been investigated for a vanadium and nitrogen microalloyed low carbon bainitic steel. Finer lath bainite microstructure can be obtained in the plate with relaxation. The results of quantitative statistics show that in the plate 80% of the total bainite lath bundles are in the range 5-15 μm in length and 3-13 μm in width, while in the plate with relaxation 80% of the total bundles are in the range 3-9 μm in length and 1 -7μm in width. Mechanical properties show that the plate with relaxation has higher impact energy, yield strength and hardness than the plate without relaxation, also the comprehensive performance after tempered at 650 ° C is superior to the plate without relaxation.