Cellulose nanowhiskers (CNW) have large potential to be used as polymeric nanocomposites reinforcement because have Young’s modulus around 150 GPa. It is also easier to obtain reinforced nanocomposites by casting due to the good dispersion of the CNW in the polymeric solution liquid than by melting. Usually a surface modification of the CNW is done to improve the mixture with the polymeric matrix. In this project, the CNW were treated with an oligomer obtained by the reaction between 4,4’-methylene diphenyl diisocyanate (MDI) with oleic alcohol to improve the CNW mixture in a bio-based polyurethane thermoplastic (TPU). Two techniques were used to prepare the nanocomposite. First the CNW were reacted with MDI during 15 min and posteriorly this system reacted with the oleic alcohol during 1h30min. The films obtained by casting with 5%CNW (w/w) were homogeneous and had good mechanical properties, having an increase of the Young’s Modulus around 50% with the same tensile strength and strain to failure values. Also TPU/CNW nanocomposites were prepared by melting using an internal mixer . The Young´s modulus of the nanocomposite with 5%CNW (w/w) had an increase around 130% with a decrease of tensile strength and strain to failure around 20% and 50%, respectively. Even with the reduction of the strain to failure around 50%, an elongation around 320% larger than the pure TPU was observed. Therefore, the nanocomposite obtained by melting had excellent mechanical properties, increasing the range of application of this material.