We synthesized a series of rod rod block copolymers, poly(octyl-4'-(octyloxy)-2-vinylbiphenyl-4-carboxylate)-b-poly(gamma-benzyl-L-glutamate) (PVBP-b-PBLG) with different rod diameters, and we focused on the effect of size disparity on their self-assembling behaviors in bulk. By using differential scanning calorimetry, wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering, and transmission electron microscopy techniques, we found that the columnar nematic (Phi(N)) liquid crystalline phase of PVBP and the hexagonal columnar (Phi(H)) phase of PBLG were disrupted to different extents depending on the volume fractions of PVBP (f(PVBP)'s). When f(PVBP), is about 60%, the Phi(H) phase of PBLG is retained with the appearance of the two second-order diffractions in WAXD patterns. When f(PVBP) is increased to above 73%, the ordered structure of PBLG is destroyed, and when f(PVBP) is 91% even the first-order diffraction peak disappears. At the same time, the Phi(N) phase of PVBP changes from poorly ordered to well ordered with increasing f(PVBP). Accordingly, with f(PVBP) increasing from 60% to 91%, the microphase-separated structure goes through a poorly ordered structure to an interdigitated lamellar morphology and to a bilayer lamellar structure.