Polyisocyanate-functionalized graphene oxide (PI-GO) was synthesized at various functionalization reaction times of 24, 48 and 72 h. The modified GO sheets were characterized by X-ray photo electron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, thermal gravimetric analysis, and atomic force microscope. It was shown that PI polymer has been successfully grafted on the surface of GO through covalent bonding between the hydroxyl and carboxyl groups of the GO with isocyanate groups of PI. The results obtained from X-ray diffraction analysis also revealed that the interlayer distance of the GO increased after modification. The highest amount of PI grafted on the GO surface was obtained after a 72 h modification reaction. The modified and unmodified GO sheets were then incorporated into a polyurethane (PU) matrix to prepare PU/PI-GO and PU/GO composites. It was found that addition of only 0.1 wt% PI-GO significantly increased the Young's modulus, work of fracture, stress at break, and elongation at break of PU/PI-GO composite as compared to the neat PU sample and the PU/GO composite. (C) 2015 Elsevier B.V. All rights reserved.