Abstract: The isothermal relaxation experiments of the high niobium steels containing rare earth were conducted at 900 ℃ using Gleeble-1500D simulator. The microstructures, precipitation behavior of second niobium containing phase and hardness of the steel after isothermal relaxation followed by quenching were studied using laser confocal microscopy, scanning electron microscopy and Vickers hardness tester.The results show that the stress relaxation process of the experimental steel was divided into three stages of precipitation nucleation, growth and coarsening. The start and end time of second phase precipitation was 10 s and 433 s respectively. With the increase of the isothermal relaxation time, the bainite lath size decreased gradually and the lath microstructure was refined significantly. The precipitations consisted of evenly distributed spherical particles. The hardness of the experimental steel first increased then decreased with the increase of the relaxation time.