The development of ultrafine grained microstructures in austenitic stainless steel and pure titanium subjected to large strain deformation was comparatively studied. The change in the volume fractions of newly developed ultrafine grains was used to quantify the progress in grain refinement during plastic deformation. The grain refinement kinetics could be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation as a function of true strain. The grain refinement kinetics was suggested being sensitively depended on the deformation conditions and the deformation mechanisms operating during severe plastic working. Under conditions of warm working, an increase in the deformation temperature accelerated dynamic recovery and, therefore, promoted the new grain development. Under conditions of cold working, the grain subdivision in austenitic stainless steel and titanium during cold working was assisted by the deformation twinning. However, the kinetics of grain refinement in austenitic stainless steel was faster as compared to pure titanium owing to strain-induced martensitic transformation.