Ultrafine-grained steel sheets with the chemical composition of 0.15%C-0.74%Mn- 0.01%Si have been prepared using a laboratory rolling mill by Super Short Interval Multi-pass Rolling (SSMR) process, in which the inter-pass time is extremely shortened to enhance the cumulative strain. The SSMR process with a finish rolling around Ae3 leads to ultrafine equiaxed ferrite structure with 1μm in average grain size. In order to clarify the grain refinement mechanism in the SSMR process, the deformation substructure in deformed austenite was simulated using 70%Ni-30%Fe, which was a fcc alloy with equivalent stacking fault energy to C-Mn steels. TEM observations have shown that the dislocation substructure in the Ni-Fe alloy hot-rolled by SSMR process mainly consists of dislocation cells, of which size are refined to less than 1μm with shortening inter-pass time. It is concluded that the SSMR process can accumulate the deformation strain in austenite enough to densely nucleate ferrite inside austenite grains.