Abstract: The hot rolled and annealed nitrogen bearing martensitic stainless steel 420U6 (/% ：0. 16C, 13. 63Cr, 0. 083N) was cold rolled by a 4-high reversable cold rolling mill with the cold rolling reductions 10% ~ 90% , and the effect of cold rolling reduction on microstructure and mechanical properties of the steel was studied. The results show that the number of the caibides considerably increases from 0. 1 X 10⁶/mm2 at the initial annealed state to 2, 35 X 10⁶/mm2 at the 90% cold rolled state, while the average diameter of the carbides decreases from 1.11 μm to 0. 22μm  and the distribution of the cariaides become more evenly. Additionally, hardness and yield strength hugely increases from 170 HV and 335 MPa in the initial annealed state to 325 HV and 1 108 MPa at the 90% cold rolled state respectively, while the elongation reduces sharply when cold rolling reduction increases from 0% to 40% , after which the elongation gradually stabilizes at about 6%.