Two low carbon steels were hot rolled by thermo-mechanical process (TMCP), and one contains niobium (0.013%), and another does not contain niobium. By the comparison of tested results, it can be found that the yield strength of Nb steel is 55MPa higher than that of Nb free steel on an average. In addition, the ferrite grain of Nb steel is finer than that of Nb free steel under the same finishing temperature, cooling rate, and final temperature. However, there are no Nb (C,N) particles to precipitate from austenite and ferrite by the observation of transmission electron microscopy (TEM). It can be concluded by theoretical analysis that both the increase of strength and the refinement of ferrite grain of Nb steel result from solid solution Nb atoms.