Dissipative particle dynamics simulations have been performed to study the coassembly behavior of short carbon nanotubes and diblock copolymers in concentrated solutions by modeling nanotubes as cylindrical nanoparticles (CNPs). With varying the hydrophobic/hydrophilic ratio of the diblock copolymers (A(m)B(n)) at fixed concentration and fixed chain length (m + n = 1.5), the influence of the addition of CNPs on the lyotropic mesophases including micellar, hexagonal, and lamellar phases has been examined in detail, in comparison with the reference systems involving spherical nanoparticles (SNPs). The simulations reveal that the one-dimensional nature of CNPs plays an important role in determining the final coassembly superstructures. It is interesting to find that the addition of CNPs has significantly increased the stability of the hexagonal phase of coassemblies by inducing the phase transformations of the micellar-to-hexagonal in A(3)B(12) solutions and the lamellar-to-hexagonal in A(7)B(8) solutions, whereas the later morphology change is missing in the presence of SNPs. In addition, both CNPs and SNPs could be selectively accommodated into the hydrophobic layers of the lamellar phases but exhibit different spatial and orientational arrangements. The present study may provide valuable guidance for the design and fabrication of patterned polymer nanocomposites with well-dispersed and aligned one-dimensional nanoparticles.