Abstract: The effects of niobium content on the microstructure and hardness of micro-niobium H13 steel were investigated through metallographic microscope and hardness testing.The results show that the fine spheroidal carbide disperses uniformly on the ferrite matrix, and the grains are refined continuously and the microstructure is more uniform when H13 steel is annealed at 860 ℃.The hardness increases with the increase of Nb content after isothermal spheroidizing annealing at 860 ℃.The hardness of the sample with Nb content of 0.10%is highest, and the hardness reaches 74.2 HRC.