Ultrafine-grained structures formed dynamically through simple compression at warm deformation temperatures were investigated in a 0.15%C- 0.4%Si-1.5%Mn steel. The effects of strain, strain rate and deformation temperature on the microstructural evolution were examined using an isothermal plane strain compression technique with a pair of anvils. The maximum strain was 4, the deformation temperature was below the A(CI) temperature, and the Zener-Hollomon parameter (Z) ranged between 10(12) s(-1) and 10(16) s(-1). Ultrafine ferrite grains surrounded by high angle boundaries are generated by simple compression when the strain exceeded a critical value. The number of newly generated ultrafine grains increased with the strain; however, the average sizes were found to be independent of strain. The gain size, 'd', was found to depend on Z parameter. An equation, d (mu m) =10(2.07)Z(-0.16), was found to satisfy the experimentally obtained data. This study demonstrates the possibility of obtaining ultrafine ferrite through multi-pass caliber rolling as a high Z- large strain deformation technique for producing bulk engineering components. It was also noted that the empirical relation established based on single pass compression tests is valid for multi-pass caliber rolling.