Abstract: The microstructure and property of hot-rolled pickling steel plate QStE500TM before and after Mn, Nb content adjustment were studied using optical microscope, tensile testing machine and transmission electron microscope. The different strengthening mechanisms and strengthening effects were analyzed. The results show that after decreasing the mass fractions of Mn and Nb from 1.6%, 0.050% to 1.3%, 0.035%, respectively, the microstructures of tested steel plates were both composed of ferrite with the grain size of grade 13.5. Furthermore, the yield strength decreased from 621 MPa to 574 MPa and the tensile strength from 654 MPa to 611 MPa. When the mass fraction of Mn decreased by 0.3%, the solid solution strengthening effect declined 11.1 MPa. When the mass fraction of Nb decreased by 0.015%, the precipitation amount of Nb-Ti-C second phase particles dropped. The precipitation strengthening effect declined 35.9 MPa.