In order to understand the effect of grain size bimodality on the tensile properties, low-carbon steel samples having bimodal ferrite grain size distributions, with different coarse-grain sizes (15-54μm), fine-grain sizes (2.4-13.0μm) and coarse-grain fractions (0.10-0.45) have been tensile tested. Samples having bimodal grain structures showed lower strength (UTS: 395-744MPa), better tensile ductility (e u: 8.5-19.4%) and lower YS to UTS ratio (0.63-0.84) than the ultra-fined grained steel (UTS=900MPa, e u=5%, and YS:UTS=0.93). In case of mixed grain structures, effective grain size (D eff), numerical average grain size (D avg) and the average grain size in the fine-grain regions (D f) showed better correlation with strength (i.e. strength ∝ D -1/2) than the weighted average grain size (D wt). Coarse-grain sizes in the range of 15-35μm and fraction of the coarse-grain regions in the range of 15-35% can be recommended for achieving the best tensile ductility. Measurement of micro-hardness and the change in grain aspect ratios from the coarse- and fine-grain regions indicated that the bimodal grain structure can be considered as a composite structure, where stress and strain vary between the coarse- and fine-grain regions. Lower nucleation and higher growth of the voids in the carbide-depleted, coarse-grain regions (than carbide-rich, fine-grain regions) allow higher plastic deformation till fracture and improve the total-elongation of the steels. © 2012 Elsevier B.V.