Ferrite grain refinement by hot rolling mostly above Ae3 being followed by an ultra-fast cooling has been investigated. An emphasis has been paid on the interval, Δt, between the finish of rolling and the start of water spray cooling of which cooling rate is more than 1000 °C·s-1. When Δt was nearly equal to zero, ultra-fine ferrite of about 1 or 2 μm in grain diameter was obtained for 1.2 to 1.3 mm thick 0.1%C-1%Mn steel sheets near sheet surfaces or in thickness center regions respectively, although the grain size at Δt of 0.5 s was about 3 μm in both regions. The ferrite grains were almost equiaxed and surrounded by high angle boundaries. This grain refinement is likely to be caused by an increased number of nucleation sites for the transformation from austenite to ferrite due to the ultra-fast cooling. Such a grain refinement mechanism is discussed in terms of prior-austenite structures deduced by the misorientation distribution function analysis.