Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress-true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae3 temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae3 was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.