While there is a growing body of work that supports the self-assemblies of nanotubes and nanorods, little attention has been devoted to understand the relation between their length and the kinetics of self assembly in polymer composites. Using dissipative particle dynamics (DPD) method, we simulated the temporal developments of equilibrium microstructures of nanotube dispersions with a bimodal length distribution in polymer matrix. The nanotube/polymer models were developed with different sets of interactions between the components. The equilibrium morphologies obtained for nanotubes are in good agreement with those proposed by previous experimental and theoretical studies. We found that long nanotubes could self-assemble into ordered honeycomb-like bundles as validated with the structure factor calculations. The self-assembly kinetics was quantitatively estimated at different stages and length scales using the variations in the pair-correlation functions. It was observed that the kinetics slowed down particularly in the initial stages of the self-assembly. This was mainly ascribed to the spatial interferences of bidisperse nanotubes as evidenced by laser scanning microscopy and simulated mean squared-displacements (MSD). Furthermore, the developed microstructures were assessed in terms of the effective nanotube volume derived from Monte Carlo (MC) calculations and the frequency of nanotube-nanotube contacts. The simulations reported herein contribute to a microscopic interpretation of the literature results, and the findings of this paper contribute meaningfully to the design strategies aimed at achieving novel nanocomposites with optimal physical properties. (C) 2017 Elsevier Ltd. All rights reserved.