Ultrafine-grained bars of a low carbon steel with a cross section of 12.9 mm and 7.9 mm square are fabricated through a multi-pass groove rolling at 773 K. A three-dimensional explicit finite element analysis is employed to evaluate inhomogeneous equivalent strain distribution and cross-sectional shape in the rolled bars. Microstructures at the center and on some edges of the cross section in the rolled bars are observed, and the cross-sectional hardness distribution is measured for each rolled bar. The microstructure and the hardness at the sites where a large strain is introduced are clearly different from those at the sites where a small strain is introduced. The variations on the microstructures and the hardness on the cross section in a warm groove rolled bar are discussed with respect to the strain distribution that is predicted from the numerical analysis.