The hot compression tests using Gleeble 1500 were performed by varying the true strain up to 1.6 (80% reduction) in Nb-free and Nb-microalloyed steels. The effect of Nb addition on the transformation kinetics during deformation of undercooled austenite was investigated. It was found that as compared with Nb-free steel,the transformation incubation period of Nb-bearing steel was pro-longed and the transformation kinetics curves parallelly moved to higher strain because of the solute Nb drag effect. Studies on kinetics also showed that the deformation-enhanced ferrite transformation (DEFT) of the two steels were composed of three stages,which can be expressed by the J-M-A equations individually. However,the parameter n related to the mode of nucleation and growth is somewhat different in the first and second stages of the two steels,and the same in the third stage for both the steels corresponding to the nuclea-tion of retained austenite. The hot compression tests using Gleeble 1500 were performed by varying the true strain up to 1.6 (80% reduction) in Nb-free and Nb-microalloyed steels. The effect of Nb addition on the transformation kinetics during deformation of undercooled austenite was investigated. It was found that as compared with Nb-free steel, the transformation incubation period of Nb-bearing steel was pro-longed and the transformation kinetics curves parallelly moved to higher strains because of the solute Nb drag effect. Studies on kinetics also showed that the deformation However, the parameter n related to the mode of nucleation and growth is somewhat different in the first and second stages of the two steels, and the same in the third stage for both the steels corresponding to the nuclea-tion of retained austenites.