Polyurethane (PU) nanocomposites incorporated with polydopamine treated carbon nanotubes (PDA-CNTs) were prepared by solution casting method via in-situ polymerization, achieving excellent CNTs dispersion in the PU matrix. It was observed that, with a loading of only 0.3 wt%, the tensile modulus and tensile strength were increased by 40% and 100% respectively. This reinforcement even exceeded the predicted modulus by Halpin-Tsai model. It indicates extremely enhanced interfacial interaction between the fillers and the matrix. More interestingly, it was found that the incorporation of PDA-CNTs caused a significant change in the degree of phase separation in the PU matrix leading to a unique network-like structure which is responsible for the unexpected reinforcement effect. This understanding combining with the synthetic approach should be useful for future design of PU/CNTs nanocomposites for wide range of applications. (C) 2016 Elsevier Ltd. All rights reserved.