We processed a ferritic-pearlitic dual-phase steel under high pressure torsion for three and five rotations at room temperature, the results shew the concurrent processes of the grain refinement of ferrite phase and the decomposition of the cementite lamellae. After three rotations, a non-homogeneous structure was observed. The ferrite structure contained both cellular structure and banded nano-granular structure. The cementite was fragmented to fine particles, aligned along the longitudinal direction of the banded ferrite structure in the local region. After five rotations, a homogeneous mixture of nanoscaled equiaxed ferrite crystallites and cementite particles was obtained, without any visible trace of the former lamellar-type structure or particle alignment. The size distribution of the remaining cementite particles turned to a more narrow distribution skewed to finer sizes especially smaller than 10 nm, since the torsion increased from three to five rotations.